UNIVERSITY  Of  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


THE  CONTROL  OF  RED  SPIDERS 
IN  DECIDUOUS  ORCHARDS 


BY 

E.  R.  deONG 


BULLETIN  No.  347 

August,  1922 


UNIVERSITY   OF   CALIFORNIA  PRESS 

BERKELEY,  CALIFORNIA 

1922 


David  P.  Barrows,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS   OF   DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science,  Director  of  Agricultural  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 

C.  B.  Hutchison,  Director  of  the  Branch  of  the  College  of  Agriculture  at  Davis. 
H.  J.  Webber,  Subtropical  Horticulture,  Director  of  Citrus  Experiment  Station. 
William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 
John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
James  T.  Barrett,  Plant  Pathology. 
Walter  Mulford,  Forestry. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
L.  D.  Batchelor,  Orchard  Management. 
W.  L.  Howard,  Pomology. 
*Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

W.  B.  Herms,  Entomology  and  Parasitology. 
John  E.  Dougherty,  Poultry  Husbandry. 

D.  R.  Hoagland,  Plant  Nutrition. 
G.  H.  Hart,  Veterinary  Science. 

L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 

DIVISION   OF  ENTOMOLOGY   AND   PARASITOLOGY 

W.  B.  Herms  H.  H.  Severin 

C.  W.  Woodworth  E.  R.  de  Ong 

E.  C.  Van  Dyke  G.  H.  Vansell 

E.  O.  Essig  J.  F.  Lamiman 
S.  B.  Freeborn 


*  In  cooperation  with  office  of  Public  Roads  and  Rural  Engineering,  U.  8.  Department  of 
Agriculture. 


THE  CONTROL  OF  RED  SPIDERS  IN 
DECIDUOUS  ORCHARDS* 

By  E.  E.  de  ONG 


CONTENTS  page 

Introduction 40 

Life  history  notes  and  habits  of  the  three  species  of  red  spiders 40 

I  Common  red  spider  (Tetranychus  telarius) 41 

Rate  cf  oviposition 41 

Winter  habits 43 

Food  plants ? 44 

II  Brown  mite  (Bryobia  praetiosa) 45 

Variation  in  hatching  dates  of  winter  egg 46 

Food  plants 47 

III  Citrus  mite  (Paratetranychus  pilosus) 48 

Food  plants 48 

Why  red  spiders  should  be  controlled 48 

Effect  of  defoliation  on  the  prune 50 

Relation  between  irrigation  and  red  spider  injury 54 

Bud  development 54 

Dispersal  of  red  spiders 56 

Preventive  and  control  measures 58 

I  Common  red  spider  (Tetranychus  telarius) 58 

Preventive  measures :  Abundant  soil  moisture.   Selected  cover  crops 

Cultural  practices.     Intercrops 58 

Control  measures 59 

Spraying  versus  dusting 59 

Sulfur  mixtures  for  spraying 60 

Recommended  practice 60 

Preparation  of^sulfur  pastes 62 

Experimental  spraying 64 

Sulfur  dusting 66 

Sulfur  fillers 67 

Intervals  between  dusting 69 

Substitutes  for  sulfur 70 

Nicotine  dusts .' 71 

Natural  enemies 72 

II  Brown  mite  {Bryobia  -praetiosa) 72 

Winter  spraying:  Crude  oil  emulsion 73 

III  Citrus  mite  (Paratetranychus  pilosus) 78 

Spray  program 78 

Cost  of  spraying  and  dusting 79 

Summary 80 

*  This  work  was  done  in  cooperation  with  members  of  the  Pomology  Division 
of  the  College  of  Agriculture,  University  of  California,  to  whom  I  am  indebted  for 
the  interpretation   of  horticultural   data.     I  also   wish  to   express  my  thanks  to 

Mr.  H.  B.  Stabler,  County  Horticultural  Commissioner  of  Sutter  County,  to  the 
orchardists  of  Sacramento  Valley  who  have  cooperated  with  me,  to  the  student 

helpers  in  this  work,  and  others  too  numerous  to  mention.     The  photographs  are 
largely  the  work  of  W.  C.  Matthews. 


40  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


INTRODUCTION 

The  so-called  Red  Spiders  are  among  the  most  persistent  and 
injurious  pests  of  California's  horticulture,  yet  no  other  pests  are 
more  frequently  allowed  to  go  unchecked.  The  loss  which  they  cause 
is  frequently  not  noticed,  for,  unlike  apples  attacked  by  the  codling 
moth,  the  fruit  of  prune  and  peach  trees  injured  by  red  spider  is 
marketable.  The  grower's  loss,  however,  is  just  as  real,  but  comes 
in  so  subtle  a  way  as  to  be  overlooked  by  many.  Studies  of  yields 
and  of  bud  development,  in  attacked  and  in  normal  orchards,  have 
shown  an  annual  crop  loss  ranging  from  $100  to  over  $400  per  acre. 
Besides  this  immediate  loss  there  is  the  injury  to  the  buds  that  will 
produce  next  year's  crop.  These  are  so  weakened  that  they  may 
shatter  off,  or  at  least  produce  infertile  bloom,  while  the  sap  is  so 
thinned  that  the  tree  may  become  more  susceptible  to  frost.14 


LIFE    HISTORY    NOTES    AND    HABITS    OF  THE   THREE    SPECIES    OF 

RED   SPIDERS 

There  are  three  species  of  plant  mites,  commonly  called  "red 
spider, ' '  which  have  long  been  recognized  as  more  or  less  serious  pests 
of  deciduous  orchards,  viz : 

(I)  Tetranychus  telarius  Linn,*  the  species  active  through  mid- 
summer and  fall,  generally  known  as  the  two-spotted,  summer,  or 
common  red  spider. 

(II)  Bryobia  praetiosa  Koch,f  the  brown  or  almond  mite,  pri- 
marily of  the  almond  and  prune,  attacks  during  spring  and  early 
summer. 

(III)  Paratetranychus  pilosus  Can.  &  Fanz.,$  commonly  found  on 
citrus  and  deciduous  trees  through  spring  and  summer. 


*  Tetranychus  telarius  Linn,  and  T.  bimaculatus  Harvey  are  considered  synony- 
mous, while  T.  sexmaculatus  Riley  is  considered  a  distinct  species.  The  first  two 
are  commonly  found  on  deciduous  trees  and  other  plants  throughout  the  state,  and 
T.  sexmaculatus  (yellow  mite)  is  usually  found  on  citrus  trees. 

t  Bryobia  praetiosa  Koch,4  synonym  Bryobia  pratensis  Garman. 

X  Paratetranychus  pilosus  Can.  &  Fanz.,  synonyms  Tetranychus  mytilaspidis 
Riley,  T.  citri  McG.  Garman  recently  called  attention  to  the  appearance  of  a  new 
mite  in  Connecticut.^  On  comparison  of  this  species  with  our  citrus  mite  they 
seemed  so  similar  that  both  Ewing  and  Quayle  have  expressed  the  belief  that  the 
two  species  may  be  considered  identical.  McGregor,  however,  considers  the  two 
species  distinct. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


41 


I.     COMMON  BED  SPIDER  (Tetranychus  telarius  Linn.) 

The  common  red  spider  is  a  web-spinning  species,  usually  found 
on  the  under  side  of  the  leaf,  excepting  on  the  almond  tree,  where  the 
mite  feeds  on  either  leaf  surface.  This  mite  may  be  found  in  the  adult, 
and  even  in  the  egg  and  nymphal  stages,  in  almost  any  month  of  the 
year  in  the  southern  part  of  Sacramento  Valley.  The  spring  and  early 
summer  months  are  spent  on  weeds  and  hardy  cultivated  plants,  such 
as  strawberries  and  violets.  Migration  to  orchard  trees  occurs  at  the 
death  or  maturity  of  the  spring  host  plant.  During  the  middle  and 
late  summer  the  mites  increase  rapidly  and  cause  severe  and  pro- 
tracted injury. 

TABLE  I 

Rate  of  Oviposition  of  Red  Spider  (T.  telarius) 
Number  of  eggs  from  different  mites. 


Date 

Max. 
Temp. 

Mite 
No.  1 

Mite 
No.  2 

Mite 
No.  3 

Mite 
No.  4 

Mite 
No.  5 

June  30 

83°  F. 

3 

July     1 

85 

4 

July     2 

72 

6 

July     3 

82 

6 

July     4 

90 

July 

88 

14 

July     6 

83 

14 

July     7 

79 

8 

16 

16 

13 

July     8 

80 

4 

3 

13 

8 

July     9 

79 

2 

16 

6 

4 

July   10 

76 

16 

16 

13 

July   11 

77 

9 

2 

July   12 

70 

4 

2 

Total 



33 

28 

64 

51 

42 

42  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Egg. — The  egg,  barely  visible  to  the  naked  eye,  is  generally  found 
on  the  under  surface  of  the  leaf  or  scattered  through  the  web  attached 
to  the  leaf.  It  is  round,  and  when  first  deposited  almost  colorless,  but 
becomes  yellowish-red  before  hatching.  The  rate  of  deposition  of  eggs 
is  dependent  upon  food  and  temperature ;  temperature  also  influences 
the  time  required  for  hatching.  Ovipositing  is  rare  at  maximum  daily 
temperatures  of  40°-50°  P.,  but  beyond  this  the  rate  rises  rapidly, 
the  highest  being  at  maximums  of  76°  F.  to  about  100°  F.  At  the 
latter  temperatures,  12  to  16  eggs  per  day  were  frequently  deposited, 
but  at  daily  maximum  temperatures  of  70°  to  75°  F.  the  daily  rate 
was  from  2  to  8.  At  a  mean  maximum  temperature  of  87°  F.  the 
duration  of  the  egg  stage  was  3  days,  and  a  drop  to  a  mean  of  77°  F. 
lengthened  the  average  of  the  egg  stage  to  6  days. 

The  totals  do  not  indicate  the  entire  number  of  eggs  which  a  single 
individual  would  normally  deposit  but  are  records  of  a  few  days  only. 
Other  investigators4  have  found  total  egg-laying  periods  of  12  to  36 
days  and  a  total  number  of  eggs  per  female  mite  ranging  from  51 
to  110.8 

The  variations  in  rate  associated  with  the  above  temperature 
changes  are  in  accordance  with  Ewing's  results  in  Oregon  during  the 
month  of  October.4  Temperature  records  were  not  given  in  Ewing's 
report  but  probably  were  not  above  72°  F.  The  maximum  number 
of  eggs  deposited  daily  under  these  conditions  was  nine  and  the  mini- 
mum one,  the  average  being  two  and  three  twenty-seconds. 

The  duration  of  the  egg  stage  is  directly  associated  with  the  tem- 
perature, as  is  shown  in  Table  II.  At  a  mean  maximum  temperature 
of  87°  F.  the  length  of  the  egg  stage  is  3  days;  a  mean  maximum 
temperature  of  77°  F.  lengthened  the  egg  stage  to  6  days.  The  num- 
bers of  eggs  used  are  too  small  to  give  reliable  data,  but  they  agree 
with  McGregor's  and  Donough's  results  in  South  Carolina  where 
larger  numbers  were  used.8 

Larva. — The  newly  hatched  larva  is  almost  transparent  and  color- 
less but  after  it  begins  to  feed  it  changes  to  a  greenish  color.  It  is 
six-legged  in  this  first  stage  and  has  a  round  body.  Movement  over 
the  leaf  surface  is  very  restricted. 

Nymphal  stages. — The  mite  has  eight  legs  beginning  with  the  first 
molt.  In  the  older  stages  the  mites  feed  voraciously  and  wander  about 
freely.  The  time  of  development  from  the  egg  to  adult  is  from  five 
to  ten  days  at  summer  temperatures. 

Adult. — The  adult  mite  moves  rapidly  over  the  leaf  surface  and 
the  webbing.    The  female  is  larger,  with  a  body  more  nearly  rounded 


BULLETIN   347]       CONTROL  OF   RED  SPIDERS   IX   DECIDUOUS   ORCHARDS 


43 


than  the  male's.  Both  are  so  small  as  to  be  distinguished  only  with 
difficulty  by  the  naked  eye.  The  adult  is  yellowish-green,  usually 
with  one  large  irregular  dark  spot  or  a  cluster  of  small  ones  on  each 
side  of  the  upper  surface  (see  Fig.  1,  opposite  page  44).  The  colors 
of  the  common  red  spider,  as  shown  in  the  figure,  were  chosen  as 
typical  for  the  adult  while  feeding.  These  may  vary  with  different 
host  plants,4  but  the  most  striking  change  results  from  starvation. 
Mites  which  are  hibernating  or  insufficiently  fed  usually  assume  a 
yellowish  or  red  color. 

TABLE  II 
Duration  of  the  Egg  Stage  of  the  Common  Red  Spider  (T.  telarius) 


Length  of  egg  stage  Mean  max.  temp. 


July  3 
July  4 
Julv    5 


Eggs  deposited 
Eggs  hatched 


3  davs 


87°   F. 


July    9 

Eggs  deposited 

July  10 

Julv  11 

July  12 

6  days 

77°    F. 

July  13 

July  14 

Eggs  hatched 

July  17 

Eggs  deposited 

Julv  18 

Julv  19 

Julv  20 

6  davs 

77°    F. 

Julv  21 

July  22 

Eggs  hatche  1 

Winter  habits. — The  common  red  spider  passes  the  winter  in  the 
adult  and  nymphal  stage  on  winter-growing  plants,  or  hibernates 
among  leaves  or  in  the  soil.  Colonies  of  mites  ranging  from  eggs  to 
adults  may  be  found  on  the  leaves  of  wild  morning  glory  (Convolvulus 
arvensis)  during  the  first  part  of  December.  When  the  upper  part 
of  the  morning  glory  is  killed  by  frost,  the  mites  are  forced  to  hibernate 
in  the  soil,  on  underground  suckers  or  among  dead  leaves.  Hibernat- 
ing mites  have  also  been  found  in  bean  straw  and  on  blackberry  and 
loganberry  leaves  where  the  canes  were  lying  on  the  ground.  Con- 
trary to  common  belief,  the  writer  has  never  found  them  underneath 
the  bark  of  trees.  It  is  probable  that  those  attempting  to  hibernate 
in  such  situations  would  fall  a  prey  to  the  predators  that  are  usually 
found  there. 


44  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Hibernation  experiments. — To  determine  the  location  of  hibernat- 
ing mites  during  midwinter,  soil  from  beneath  plants  which  were 
infested  in  the  late  fall  was  carefully  removed,  layer  by  layer,  to  a 
depth  of  four  inches  and  kept  in  a  warm  room.  Each  lot  was  placed 
in  a  separate  jar,  the  top  edge  of  which  was  covered  with  tanglefoot 
to  prevent  any  mites  entering  from  the  outside.  Seeds  or  uninfested 
young  seedlings  of  beans  or  other  host  plants  were  planted  in  these 
jars,  which  were  held  at  a  temperature  of  60°  to  70°  F.  Infestation 
was  noted  in  almost  every  jar  within  a  few  days  after  planting,  show- 
ing that  the  mites  were  lying  dormant  in  the  soil  or  surface  mulch  of 
leaves.  The  greatest  number  were  found  on  the  immediate  surface, 
although  in  one  jar,  with  soil  from  a  four-inch  depth,  a  single  mite 
appeared.  Experiments  with  bark  from  the  trunks  of  trees  gave 
negative  results. 

Cold  storage  experiments. — Great  resistance  to  freezing  temper- 
atures was  shown  by  adults  and  the  older  nymphs  of  T.  telarius. 
Colonies  from  wild  morning  glory  were  placed  in  cold  storage  at 
temperatures  of  36°,  40°,  and  50°  P.  for  21  days,  and,  when  returned 
to  a  temperature  of  70°  P.,  living  mites  were  found  in  containers 
from  each  temperature,  the  highest  record  being  66  per  cent  from 
the  room  held  at  36°  F.  One  adult  female  mite  from  this  lot  was 
placed  on  a  young  bean  plant  and  in  two  days  had  deposited  three 
eggs,  but  she  died  on  the  fourth  day.  These  eggs  hatched  in  seven 
days  at  a  temperature  of  65°-75°  F.,  thus  proving  that  the  mite  was 
quite  normal  after  an  exposure  of  three  weeks  at  36°  F.  Four  female 
nymphs  and  one  adult  male  from  this  same  temperature  were  also 
kept  alive  for  several  days. 

A  second  and  longer  experiment  was  then  tried  with  mites  of  the 
same  species,  stored  from  November  9,  1920,  to  January  3,  1921,  at 
temperatures  of  32°,  36°,  and  45°-50°  F.  The  per  cent  of  living 
mites  found  in  the  different  series,  when  removed  to  a  warm  room, 
was  8,  12,  and  5,  respectively. 

In  most  localities  in  California  where  red  spiders  are  serious  pests, 
it  is  probable  that  dormancy  will  alternate  with  periods  of  activity 
during  the  winter.  A  few  eggs  may  even  be  deposited  on  warm  days, 
but  no  material  increase  occurs  until  spring. 

Food  plants. — The  list  of  recorded  host  plants  of  this  mite  is  grow- 
ing constantly;  the  following  arc  subject  to  more  or  less  severe  injury 
in  California:  almond,  apricot,  bean  (pink,  whites,  etc.),  blackberry 
(wild),  cherry,  cucumber,  geranium,  hop,  Malva  parviflora,  melon, 
morning  glory,  grapevine,  pea,  peach,  pear,  plum,  poplar,  pumpkin, 


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Bulletin  347]      control  of  red  spiders  in  deciduous  orchards         45 

rose,  squash,  strawberry,  violet,  walnut  (English  and  Northern  Cali- 
fornia Black),  watermelon. 

In  addition  to  this  list,  the  following  host  plants  have  been 
recorded  in  California  and  throughout  the  world :  abutilon,  acacia, 
alder  (red),  alfalfa,  Angelica,  apple,  arborvitae,  ash,  aster,  beet 
(sugar),  birch,  Bouvardia,  cabbage,  calla,  carnation,  castor  bean, 
celery,  chard,  chrysanthemum,  Cinchona,  citrus,  clematis,  Clitoria, 
clover  (red  and  white),  corn,  cotton,  cowpea,  cosmos,  currant,  cypress- 
vine,  dahlia,  eggplant,  elm,  fever-few,  flax,  fuchsia,  Godelia,  goldenrod, 
gooseberry,  heliotrope,  hemp,  Hevea.  holly-hock,  hop-tree,  horse-chest- 
nut, iron-weed,  jimson  weed,  larch,  lettuce,  lily  (Easter),  Manettia, 
Manihot,  maple,  maize,  mignonette,  monkey-flower,  moonflower,  mus- 
tard, okra,  onion,  passion-vine,  Papaya,  pea  (sweet),  peanut,  pecan, 
pepper,  phlox,  pink,  pigweed,  potato  (Irish  and  sweet),  radish,  rasp- 
berry, sage,  Sesoania  acgjjptiaca,  smilax,  snowball,  sunflower,  Thun- 
bergia,  tomato,  turnip,  verbena,  willow. 

In  addition  to  the  above,  McGregor  mentions  83  host  plants  that 
are  native,  wild  species  in  South  Carolina,8  perhaps  none  of  which 
is  included  among  the  above  names.  A  systematic  search  of  native, 
California,  host  plants  which  are  at  least  occasionally  infested  would 
no  doubt  add  many  plants  to  the  above  record. 

II.     BROWN  MITE  (Brijobia  praetiosa  Koch) 

The  brown  mite  is  a  non  web-spinning  species  which  winters  in 
the  egg  stage.  The  egg  hatches  as  the  buds  open  in  the  spring  and 
the  young  mite  feeds  on  the  growing  leaves  and  the  blossom.  The 
mites  usually  congregate  on  the  young  twigs,  especially  the  leaf  scars, 
through  the  day  and  migrate  to  the  leaves  at  night ;  the  other  two 
species  are  found  almost  exclusively  upon  the  leaf  blade.  There  is 
no  distinct  migration  from  one  type  of  host  to  another,  as  with  the 
common  red  spider,  the  entire  year  usually  being  spent  on  one  host. 
They  may,  however,  crawl  from  plant  to  plant  or  be  carried  about 
by  the  wind. 

Egg. — The  egg  is  deep  red,  frequently  with  a  yellowish  tinge, 
round,  and  barely  visible  to  the  naked  eye.  Eggs  may  be  deposited 
singly,  but  if  abundant,  are  usually  grouped  in  conspicuous  patches 
on  the  underside  of  the  limb.  A  favored  situation  for  ovipositing  is 
the  slightly  roughened  bark  of  two  or  three  year  old  wood.  They  are 
also  common  on  fruit  spurs,  and  leaf  and  fruit  scars.  The  white  shells 
of  hatched  eggs  and  molted  skins  of  young  mites  are  nearly  always 
found  among  the  eggs,  as  seen  in  Pig.  2. 


46 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  hatching  of  the  winter  egg  varies  with  the  host  and  is  so  inti- 
mately associated  with  the  development  of  the  spring  growth  as  to 
imply  some  relationship  between  the  activity' of  the  developing  host 
and  the  hatching  time  of  the  egg. 


TABLE  III 

Hatching  Dates  of  the  Winter  Egg  of  the  Brown  Mite  (Bryonia  praetiosa) 


Host 

Hatching  Date 

Bud  Development 

A'mond 

Feb.  21-Mar.    1,  1920 

One-third  to  full  bloom.  Leaf  clusters 
one-fourth  to  one  inch  long. 

Plum  (Grafted  on 
peach) 

Mar.  24-Mar.  31,  1920 

Bloom  buds  opening.  Leaf  clusters 
three-fourths  of  an  inch  long. 

Apple 

Mar.  27-April    3, 1920 

Cluster  buds  opened.  Leaf  clusters  one- 
third  of  an  inch  long. 

The  last  generation  of  mites  on  the  apple  may  complete  their  egg- 
laying  from  two  to  four  weeks  later  than  on  the  almond,  i.e.,  July 
to  August  15.  Mites  are  occasionally  seen  in  September,  but  females 
are  rare  by  the  first  of  August.  Hence  a  period  of  from  seven  to 
eight  months  is  spent  in  the  winter  egg  on  the  apple.  Egg-laying 
on  the  almond  in  the  vicinity  of  Davis  is  usually  completed  by  July  1, 
but  on  early  defoliated  trees  this  date  may  be  as  early  as  the  last 
of  May.  The  winter  egg  stage  on  prematurely  defoliated  almond  trees 
may  continue  from  eight  to  nine  months,  but  on  normal  trees  the 
period  would  be  about  one  month  shorter. 

Larva. — Bright  red,  body  almost  globular,  six-legged.  Contrary 
to  the  habits  of  the  larval  form  of  the  common  red  spider,  it  is  very 
active,  moves  about  freely  over  the  leaf-surface  and  usually  retreats 
to  a  roughened  part  of  the  bark  through  the  day. 

Nymph. — All  stages  of  the  mite  after  molting  the  first  time  are 
eight-legged.  The  color  changes  from  red  to  brown  or  greenish  after 
the  mite  has  fed  a  short  time.  The  complete  life-history  was  not 
determined,  but  under  most  conditions  three  to  four  days  were  spent 
in  each  instar.  Nymphs  of  the  second  and  third  instar  were  found 
on  March  6,  but  no  hatching  of  the  winter  egg  was  observed  prior  to 
February  20. 

Adult. — Reddish-brown,  with  a  green  tinge.  First  pair  of  legs 
greatly  elongated.    Body  flattened.    Male  slightly  smaller  and  with  a 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  47 

mor^  pointed  body  than  the  female.  Adults  of  the  first  generation 
are  found  from  March  20  to  April  5.  Second  generation  matures  by 
April  16  and  the  third  generation  by  May  10. 


Fig.  2. — Eggs  of  brown  mite   (Bryobia  praetiosa)   on  rough  bark.     The  white 
objects  are  molted  skins  and  egg  shells.     (X  10.) 


Food  plants. — The  list  of  host  plants  for  this  mite  grows  constantly 
as  does  that  for  the  common  red  spider.  The  following  are  cultivated 
plants  subject  to  more  or  less  serious  attacks  in  California :  almond, 
prune,  plum,  peach,  pear  and  apricot. 

Those  reported  in  the  eastern  United  States,  Europe,  and  Australia 
are:  alfalfa,  barley,  buckwheat,  cherry,  cherry  (sand),  citrus  trees, 
clover,  gooseberry,  grasses,  oats,  peas,  quince,  wheat. 


48 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


III.     CITEUS  MITE  (Paratctranychus  pilosus  Can.  and  Fanz.) 

The  citrus  mite  spins  a  web,  but  less  abundantly  than  the  common 
red  spider.  The  life-history  of  this  mite  on  deciduous  trees  is  similar 
to  that  of  the  brown  mite  in  that  the  winter  is  spent  in  the  egg  stage 
on  the  host  of  the  previous  summer.  A  full  account  of  its  life-history 
on  citrus  trees  has  been  worked  out  by  Quayle.10 


Fig.  3. — Eggs  of  the  citrus  mite    (Paratetranychus  pilosus),  showing  strands 
of  webbing  and  stalk;  greatly  enlarged. 

This  mite  is  of  a  deep  red  color,  with  conspicuous  white  spines 
arising  from  prominent  tubercles  on  the  body.  The  egg  is  slightly 
striated,  a  flattened  sphere  in  shape,  with  a  prominent  stalk  about 
twice  the  diameter  of  the  egg  in  height.  This  stalk  is  bent  over  at  the 
tip  and  ends  in  a  slight  enlargement  (see  Fig.  3).  Strands  of  webbing 
frequently  extend  from  the  upper  part  of  the  stock  to  the  leaf -surface 
or  to  other  strands. 

Food  plants. — This  mite  feeds  principally  on  citrus  trees  but  is 
found  on  almond,  pear,  apple,  peach,  prune,  and  plum  trees  and  on 
evergreens,  such  as  Pittosporum,  in  northern  California. 


WHY    RED    SPIDERS    SHOULD    BE    CONTROLLED 

The  red  spider,  while  feeding,  extracts  the  sap  and  reduces  the 
amount  of  green  coloring  matter  (chlorophyll)  as  is  evidenced  by 
the  pale  color  of  the  leaves.    This  causes,  first,  a  mottled  appearance, 


Bulletin   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  49 


Fig.  4.- 


-Sample  I.  Dried  prunes  from  normal  trees  (top)  and  defoliated  trees 
(bottom).  This  fruit  was  all  grown  in  the  same  orchard  and  under  similar  con- 
ditions, except  that  one  part  of  the  orchard  was  defoliated  by  red  spider. 


50  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

and,  finally,  a  paling  of  the  entire  leaf  surface,  which  is  frequently 
followed  by  defoliation.  Restoration  from  this  type  of  injury  to  a 
normal  condition  is  rapid  if  the  mites  are  destroyed  before  the  leaves 
are  badly  damaged  (see  Fig.  10).  This  recovery  has  been  more  pro- 
nounced after  attacks  by  the  brown  mite  than  by  the  common  red 
spider,  probably  because  the  former  is  more  exposed  and  hence  readily 
killed  by  contact  sprays.  The  seriousness  of  the  injury  is  due  to  the 
fact  that  substances  are  found  in  the  leaf,  associated  with  the  green 
coloring  matter,  by  the  aid  of  which  the  plant  manufactures  sugar 
and  starch  from  which  the  plant  itself  is  developed.  These  materials 
constitute  more  than  four-fifths  of  the  dry  weight  of  all  fruit  and 
even  of  the  tree  itself.  If  this  green  coloring  matter  is  destroyed,  the 
plant  is  unable  to  feed  a  growing  fruit  crop  properly  or  to  mature 
a  normal  number  of  strong  buds  for  the  coming  year.  The  tree  which 
in  midsummer  has  pale  yellow  leaves  or  is  prematurely  defoliated  has 
suffered  a  very  serious  reduction  in  its  producing  powers  even  though 
it  may  mature  a  small  fruit  crop.  The  fruit  must  of  necessity  be  of 
small  size,  with  an  unduly  large  amount  of  pit  in  proportion  to  pulp — 
a  grade  which  is  always  a  drug  on  the  market.  Trees  that  have  been 
prematurely  defoliated  by  red  spiders  or  other  causes  and  then  irri- 
gated in  the  late  summer  will  usually  throw  out  new  leaves  (see 
Fig.  7,  d)  and  possibly  bloom,  from  buds  developed  for  next  year's 
growth.  Other  buds  which  have  not  been  forced  into  foliage  may 
have  so  much  of  their  stored  starch  withdrawn  that  they  will  shatter 
off.  A  late  second  crop  of  leaves  will  manufacture  and  store  more 
sugar  and  starch  but  it  is  doubtful  if  it  can  restore  a  tree  to  normality 
or  develop  fruiting  buds. 

The  foliage  must  be  retained  in  a  normal  condition  until  the  last 
of  September  or  the  first  of  October  to  enable  deciduous  fruit  trees 
to  yield  maximum  returns.  Foliage  injury  in  any  or  all  parts  of  the 
tree  reduces,  in  proportion  to  the  extent  of  the  injury,  the  amount  of 
food  which  the  tree  can  manufacture,  as  shown  by  Harvey  and  Mur- 
neek7  and  other  investigators.  Such  weakness  is  manifest  in  many 
ways :  by  scanty  foliage,  probably  resulting  in  sunburn,  infertile 
bloom,  and  weakened  growth.  Common  orchard  troubles,  usually 
attributed  to  the  need  of  fertilizers,  irrigation,  and  cross-pollination, 
may  in  some  instances  be  due  to  the  injury  caused  by  red  spiders, 
and  should  be  studied  from  this  standpoint. 

Effect  of  defoliation  on  the  prune. — Data  on  the  red  spider  injury 
to  the  prune  crop  were  obtained  through  the  assistance  of  H.  P. 
Stabler,  County  Horticultural  Commissioner  of  Sutter  County.  He 
collected  average,  orchard  samples  during  the  last  two  years  from 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IX  DECIDUOUS  ORCHARDS  51 

which  the  following  grades  and  weights  were  determined.  The  first 
sample  contained  300  primes  and  the  second  400.  The  two  types  of 
fruit  in  each  sample  were  grown  under  the  same  conditions,  except 
that  of  defoliation  by  red  spider  in  a  part  of  the  orchard.  Sample  I 
had  been  dried  at  the  orchard,  sample  II  was  received  in  a  fresh  con- 
dition and  afterwards  dried. 


TABLE  IV 

Comparative  Weights 

of  Fresh  and  Dried  PRr 

xes  from  Normal  axd 

, 

Defoliated  Trees 

Average  weight    Average  weight 
of  fresh  fruit         of  dried  fruit 

Xumber  of 
of  fruits  to  lb. 

Percentage 
of  pits 

ounces                    ounces 

% 

Sample  I 

Normal  

0.403 
0.22  ' 

40 
72 

11.4 

Defoliated 

13.8* 

Sample  II 

Normal 

0.565 

0.259 

64 

13 

Defoliated 

0.438 

0.205 

78 

17 

*  The  small  proportion  of  pits  found  in  sample  I  indicates  a  better  quality  of 
fruit  than  that  in  sample  II,  which  may  account  for  the  difference  in  the  effect 
on  the  two  grades. 

The  trees  defoliated  by  red  spider  in  sample  I  showed  a  crop  loss 
of  44  per  cent  by  weight  and  a  drop  of  3  points  in  the  grading.  At 
this  rate  of  decrease  a  crop  of  dried  prunes  (40-50  grade)  of  3  tons 
per  acre  would  have  lost  in  weight  1%0  tons.  The  remaining  1%0 
tous  would  have  depreciated  in  value  from  11%  cents  per  pound  for 
40-50 's  to  744  cents  per  pound  for  70-80 's,  a  total  loss  of  approxi- 
mately $453  per  acre.  Sample  II  showed  a  total  crop  loss  from  red 
spider  injury  of  $139.18  per  acre  (on  the  basis  of  three  tons  of  dried 
fruit  per  acre"),  including  both  the  reduction  in  weight  and  the 
depreciation  in  value  of  the  remainder.  The  injury  to  next  year's 
buds,  also  due  to  the  work  of  red  spider,  causes  a  further  loss. 

A  photograph  of  the  dried  prunes  of  sample  I  is  shown  in  Figure  4 ; 
the  six  at  the  top  are  from  normal  trees.  The  proportionate  number 
of  the  different  sized  fruits  in  Figure  5  is  roughly  indicated  by  the 
number  of  individual  prunes.  For  example,  ten  out  of  every  hun- 
dred fruits,  from  the  defoliated  trees,  were  shriveled  like  the  one  at 
the  left  of  row  three  from  the  top.  The  four  to  the  right  of  this  one 
indicate  the  high  proportion  of  low-grade  fruit  from  the  part  of  the 
orchard  defoliated  by  red  spider.     These  trees  had  been  absolutely 


52 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


/ 


2 


3 


4 


o 


6 


Fig.  5. — Sample  II.  Fresh  prunes  from  normal  trees  (top)  and  defoliated 
trees  (bottom)  (see  Fig.  6).  This  fruit  was  all  grown  in  the  same  orchard  and 
under  similar  conditions  except  that  one  part  of  the  orchard  was  defoliated  by  red 
spider. 


BULLETIN   3-17]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  53 


Fig.  6. — Normal  and  defoliated  prune  twigs  showing  the  condition  of  the 
trees  from  which  the  fruit  of  Fig.  5,  Sample  II,  was  gathered.  The  fruit  and 
twigs  at  the  top  of  Figs.  5  and  6  are  from  normal  and  the  lower  ones  from  defoli- 
ated trees. 


54 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


defoliated  by  red  spider  three  to  four  weeks  before  harvest,  while  the 
remainder  of  the  orchard  still  retained  a  fair  amount  of  foliage.  The 
entire  orchard  was  dry  and  in  poor  condition.  A  small  proportion  of 
large  prunes  will  be  noted  in  the  fresh  fruit  sample  but  none  in  the 
dried;  this  may  have  been  caused  by  varying  conditions  in  the  two 
orchards  from  which  the  samples  were  drawn,  or  a  low  sugar  content 
in  the  large  prunes  from  defoliated  trees  may  have  resulted  in  a 
greater  drying  loss. 

RELATION    BETWEEN    IRRIGATION    AND    RED    SPIDER    INJURY 

Attack  by  red  spider  and  drought  have  long  been  associated  together. 
In  certain  districts,  the  only  protective  measure  used  is  abundant 
irrigation,  a  practice  that  is  not,  however,  entirely  adequate  in  many 
instances.  But  irrigation  must  always  be  considered  of  primary  im- 
portance, for  an  orchard  suffering  from  drought  is  already  in  an 
unthrifty  condition  which  may  be  intensified  by  an  uncontrolled 
attack  of  red  spider.  Spraying  is  a  waste  of  money  or  at  best  of 
slight  advantage  unless  an  adequate  supply  of  moisture  is  assured. 

Bud  development. — Much  of  the  experiment  work  at  the  Univer- 
sity Farm  was  with  almonds,  and  since  the  orchard  bore  irregularly, 
it  was  necessary  to  judge  results  by  a  study  of  the  development  of 
buds  rather  than  of  production.  Buds  were  selected  in  October  in 
certain  sprayed  and  irrigated  plots,  to  determine  their  relative  devel- 
opment under  different  types  of  treatment.  The  methods  of  treating 
the  various  plots  and  the  percentage  of  moisture  found  October  9  in 
the  first  six  feet  of  soil  are  shown  in  Table  V. 


TABLE  V 

Description  of  Sprayed  and  Irrigated  Almond  Plots 


No. 

Date  of 
Treatment 

A 

June  10 

B 

June    8 

C 

June  19 

D 

June    8 

E 

June    5 

F 
G 

July  20 

14-inch  irrigation — no  spraying 

Sprayed  with  lime  sulfur   solution   2  gallons,  sulfur  5 

pounds,  dry  prepared   flour   paste   2   pounds,  in  100 

gallons  of  mixture '. 

Sprayed  with  crude  oil  emulsion  15  gallons  of  oil  in  100 

gallons  of  mixture 

Sprayed  with  5  pounds  of  sulfur,  plus  2  pounds  of  dry 

prepared  flour  paste  in  100  gallons  of  mixture  

Dusted  with  sulfur 

6-inch  irrigation — no  spraying 

Untreated 


11.51 

8.81 

t 

t 

12.23 

7.78 


fSimilar  to  plot  B. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  55 

The  efficiency  and  more  details  as  to  preparation  and  use  of  the 
spray  mixture  used  in  plots  B,  C  and  D  will  be  found  in  the  discussion 
of  Tables  VI  and  VIII.  It  should  be  noted  that  the  control  in  plot  C 
is  for  the  brown  mite,  no  attempt  being  made  to  check  the  red  spider 
attack  during  midsummer.  In  plots  B  and  D  the  application  was  made 
too  late  to  give  much  protection  against  the  brown  mite,  but  it  did 
control  the  common  red  spider.  An  irrigation  of  approximately  six 
inches  of  water  was  given  in  July  to  plots  B,  C,  D,  E,  and  F.  From 
the  contour  of  the  ground  it  was  impossible  to  apply  a  uniform  depth 
of  water  over  all  the  plots  and  judging  from  the  amount  of  soil  moist- 
ure present  in  October,  plot  F  received  considerably  more  than  the 
average  depth.  The  comparative  data  given  are  possible  from  the  acci- 
dental joining  of  irrigation  and  spraying  plots.  Plot  A  was  chosen  as 
a  typical  well-irrigated  plot  from  a  series  of  experiments  planned  by 
the  Division  of  Irrigation  Practice. 


TABLE  VI 

Comparative  Numbers   and  Weights   of  Buds   from   Sprayed  and   Irrigated 

Almond  Plots  in  October 

(Orchard  attacked  by  both  the  brown  mite  and  the  common  red  spider.) 


Plot 
No. 

Type  of  treatment 

Total  No. 
of  buds 
on  spurs 

Buds  per 
spur 

Rank  by- 
No.  of 
buds 

Total 

weight 

of  buds 

on  spurs 

Weight 
per  bud 

Rank  by 
bud  size 

Rank  by 

average 

of  weight 

and  No. 

A 

Early  summer 
irrigation 

336 

3.36 

1 

1.6429 

4.89 

1 

1.0 

B 

Slimmer  spray 
(Lime-sulfur) 

254 

2.54 

3 

1.1221 

4.42 

2 

2.5 

C 

Winter  spray 
(crude  oil) 

275 

2.75 

2 

1.1752 

4.27 

3 

2.5 

D 

Summer  spray 
(sulfur) 

177 

1.77 

5 

0.7133 

4.003 

4 

4.5 

E 

Summer  dust 
(sulfur) 

191 

1.91 

4 

0.5281 

2.76 

6 

5.0 

F 

Late  summer 
irrigation 

55 

0.55 

6 

0.1834 

3.33 

5 

5.5 

G 

Untreated 

38 

.38 

7 

0.0846 

2.23 

7 

7.0 

The  selection  of  buds,  for  the  data  given  in  Table  VI,  was  made 
as  follows :  Ten  fruit  spurs  were  selected  from  each  of  ten  trees  in 
the  different  plots,  the  buds  were  then  removed  from  these,  counted, 
and  weighed.  The  total  number  of  buds  from  the  ten  spurs  taken 
from  each  plot  is  given  in  column  three  of  Table  VI,  the  average  num- 
ber in  column  four,  while  in  column  five  the  rank  or  order  according 
to  the  total  number  of  buds  as  listed.     That  is,  the  plot  showing  the 


56  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

highest  number  of  buds  ranks  first  in  column  five.  The  same  plan  is 
followed  in  grouping  the  data  for  the  weights.  The  bud  development 
on  the  first  of  October  is  seen  with  typical  foliage  in  Figure  7. 

It  should  be  noted  that  this  comparison  is  not  alone  between  spray- 
ing and  irrigation  but  also  shows  comparative  control  values  for  two 
species  of  mites,  the  brown  mite,  and  the  common  red  spider.  Plot 
C  represents  the  control  of  the  brown  mite  alone ;  in  plots  B,  D,  and 
E,  this  mite  had  almost  ceased  feeding  for  the  year,  hence  the  value 
of  the  treatment  applies  almost  entirely  to  the  control  of  the  common 
red  spider. 

From  the  data  in  Table  VI  it  will  be  seen  that  the  trees  were 
suffering  from  a  combination  of  drought  and  red  spider  injury,  as 
irrigation  at  the  proper  time,  but  without  spraying,  resulted  in  even 
better  bud  formation  than  where  the  mites  were  controlled  but  where 
the  trees  suffered  from  drought.  The  combined  injury  from  drought 
and  red  spider  resulted  in  the  feAvest  number  of  buds,  as  shown  in 
plot  G.  The  next  lowest  in  rank  was  plot  F.  The  latter  had  been 
defoliated  rather  early  in  the  summer  and,  when  irrigated,  sent  out 
a  heavy  foliage  at  the  expense  of  next  year 's  buds ;  many  of  the  latter, 
which  had  been  formed  earlier  in  the  year,  withered  and  dropped  off 
at  this  time.  This  plot  had  practically  no  bloom  the  following  spring. 
It  should  also  be  noted  that  the  summary  of  bud  development  for 
plot  C,  where  the  brown  mite  alone  was  controlled,  was  identical  with 
that  in  plot  B,  in  which  the  attack  of  the  brown  mite  was  unchecked 
except  for  a  very  short  time,  while  the  red  spider  was  thoroughly  con- 
trolled. Apparently  the  attack  of  the  brown  mite  is  as  serious  on  the 
almond  as  that  of  the  red  spider,  a  theory  which  is  in  accord  with  the 
belief  of  many  almond  growers. 

DISPERSAL   OF    RED    SPIDERS 

The  usual  ways  of  dispersal  of  the  different  species  of  red  spiders 
are:  (1)  moving  from  leaf  to  leaf;  (2)  crawling  over  the  ground; 
(3)  wind  carriage  either  of  the  individual  mite  or  of  a  falling  leaf 
bearing  a  colony  of  spiders;  (4)  accidental  carriage  by  insects  or 
birds  which  have  rested  upon  an  infested  twig  and  then  flown  to  a 
new  host. 

Wind  carriage  is  perhaps  the  most  important  of  the  four  ways 
under  California  conditions.  Tests  with  sticky  tanglefoot  paper  have 
repeatedly  shown  that  mites  may  be  carried  hundreds  of  feet  even 
by  a  light  wind.11  But  the  fact  that  fche  falling  leaf  may  be  a  carrier 
of  entire  colonies  of  mites  has  apparently  been  overlooked.  This  seems 
to  be  a  natural  means  of  infestation  and  was  adopted  as  the  basis  for 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


57 


making  cultures  in  laboratory  practice.  A  small  section  of  an  infested 
leaf,  pinned  to  another  plant,  soon  dries,  thus  forcing  a  migration  of 
the  mites  to  the  new  host. 

Migration  is  more  pronounced  with  the  common  red  spider  than 
with  the  other  two  species.  This  is  especially  noticeable  during  the 
hottest  and  driest  months  of  the  year,  July  and  August.     Voracious 


Fig.  7. — Prune  twigs  showing  bud  development  in  sprayed  and  irrigated  plots. 

(a)  Winter    application    of    crude    oil    emulsion    to    control    the    brown    mite, 
Bryobia. 

(b)  Early  summer  irrigation. 

(c)  Summer  application  of  lime-sulfur  solution,  sulfur  and  flour  paste  to  con- 
trol the  common  red  spider,  T.  telarius. 

(d)  Late  summer  irrigation,  causing  a  second  growth  of  foliage.     These  trees 
did  not  bloom  the  following  year. 

(e)  Check.     No  spraying  or  irrigation.     Note  Bryobia  eggs  on  the  spurs. 


feeding  withers  the  leaves  of  the  host  plant  and  if  it  be  those  of  a 
small  annual  the  entire  plant  soon  dies.  The  mites  then  migrate  or 
are  blown  away  by  the  wind.  Defoliation  of  tall  trees  and  the  brush 
along  levees  facilitates  wind  carriage  and  the  infestation  may  advance 
hundreds  of  feet  in  a  few  hours. 


58  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

PREVENTIVE    AND    CONTROL    MEASURES 

I.     COMMON  RED  SPIDER  (Tetranychus  telarius  Linn.) 

The  control  of  the  common  red  spider  is  frequently  considered  to 
be  dependent  alone  upon  spraying  or  dusting.  There  are,  however, 
certain  farm  practices  whose  preventive  value  should  be  recognized. 
Such  measures  alone,  however,  cannot  be  depended  upon  to  control 
the  red  spider  under  conditions  favorable  to  its  development.  But 
increasing  the  thriftiness  of  the  tree  and  delaying  somewhat  the  attack 
on  cultivated  crops  may  be  of  great  value  in  supplementing  the  general 
spray  practice.  The  postponement  of  the  attack  even  for  two  or  three 
weeks  during  midsummer  lessens  the  injury  to  any  orchard. 

Preventive  Measures 

1.  Abundant  soil  moisture. — The  advantages  of  timely  irrigation 
are  shown  on  pages  54-56. 

2.  Selected  cover  crops. — The  cover  crops  commonly  used  in  Cali- 
fornia, including  legumes,  grains,  alfilaree,  mustard,  and  the  grasses, 
are  almost  free  from  attack  by  red  spiders.  The  common  red  spider 
has  been  found  feeding  and  ovipositing  upon  bur  clover  (Medicago 
hispida  and  M.  arabica),  but  apparently  it  does  not  thrive  upon 
either  species.  Any  of  the  above-named  plants  may,  so  far  as  has 
been  observed,  be  safely  used  as  cover  crops,  where  such  practice  is 
desirable.  They  are  of  value  not  only  for  green  manuring  but  may 
prevent  the  growth  of  host  plants  of  the  red  spider. 

3.  Cultural  practices. — Early  plowing  or  other  cultural  practices 
which  kill  or  retard  the  growth  of  early  spring  host  plants,  such  as 
wild  morning  glory,  and  the  cheese  weed  (Malva  spp.),  may  aid 
materially  in  retarding  the  development  of  the  common  red  spider. 
The  mites  are  much  reduced  in  numbers  through  the  winter  but 
multiply  slowly  during  the  spring  months  on  weeds  and  cultivated 
plants.  Migration  of  the  mites  from  the  spring  to  the  summer  host, 
whether  orchards,  hops,  or  a  truck  crop,  does  not  usually  occur  until 
the  early  host  has  matured  or  succumbed  to  their  attacks.  Killing  or 
checking  the  spring  host  may  prevent  a  great  increase  of  the  mites 
and  thus  lessen  the  number  of  migrants. 

4.  Selected  intercrops. — Intercrops  for  young  orchards,  subject  to 
red  spider  attack,  can  be  chosen  with  the  view  of  minimizing  the 
attack.  If  this  is  not  done  and  very  susceptible  intercrops  are  grown 
in  the  orchard  in  regions  subject  to  attack  by  red  spider,  both  the 
orchard  and  the  intercrop  may  suffer  more  than  if  each  crop  were 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


59 


grown  alone.    Tomatoes  and  the  various  varieties  of  corn  and  sorghum 
have  been  found  safe  to  use  as  intercrops. 

The  above  remarks  concerning  host  plants  apply  only  to  the 
common  red  spider,  not  to  the  brown  mite  or  to  the  citrus  mite.  For 
the  control  of  the  latter  two  species,  winter  or  summer  spra}4ng  and 
abundant  soil  moisture  constitute  the  main  dependence. 

Control  Measures 

Spraying  versus  dusting. — Sulfur  applied  as  a  dust  has  long  been 
used  in  California  for  the  control  of  red  spiders,  but  in  recent  years 
the  practice  has  become  increasingly  unsatisfactory.  Criticism  of 
dusting  as  a  means  of  applying  sulfur  dates  back  at  least  to  1903 
when  Volck  recommended  a  potassium  sulfid  spray  as  a  substitute  for 
dusting  in  red  spider  control  in  Sutter  County.13  Sulfur  dusting  is 
more  satisfactory  in  regions  where  the  air  is  somewhat  humid*  and 
the  winds  light — conditions  which  are  common  throughout  much  of 
the  coast  region.  Dusting  is  of  decidedly  less  value  in  the  dry,  windy, 
interior  valleys  where  the  percentage  of  relative  humidity  runs  very 
low  as  will  be  seen  in  Table  VII.  Under  such  conditions,  dependence 
must  be  placed  largely  on  spraying,  preferably  using  a  spreading  and 
adhesive  material  in  the  mixture.  These  general  deductions  have  their 
exceptions  where  the  reverse  of  the  above  climatic  features  are  found 
during  certain  seasons  of  the  year  or  in  certain  restricted  localities. 


TABLE  VII 

Monthly  Mean  of  Relative  Humidity  at  5  p.m.  During  June,  July,  and 

August,  1919  and  1920 


Location 

June 

July 

August 

San  Jose                {  1919 

(Coastal  Region)    1    1920 

% 

50 
49 
30 
30 
16 
23 

% 

56 
44 
32 
30 
16 
20 

% 

54 
51 

Sacramento          J  1919 

(Interior  valley)     1    1920 

29 
27 

PpH  "Rlnff                     1919 

17 

(Interior  valley)      1    1920 

18 

The  greatest  advantage  of  dusting  over  spraying  is  the  speed  of 
application ;  where  climatic  or  other  conditions  permit  its  use  it  is  a 
valuable  supplement  to  the  slower  work  of  spraying.     One  man  with 

*  A  distinction  must  be  made  between  the  amounts  of  condensed  moisture  on 
leaf  surfaces  relative  to  the  adherence  of  dusts.  Moisture  in  sufficient  quantity 
to  wet  the  surface  of  the  leaf  or  condense  in  drops  may  actually  prevent  the 
adherence  of  sulfur  and  cause  it  to  run  off  onto  the  lower  edge  of  the  leaf.  A 
damp  surface,  however,  will  cause  a  more  uniform  adherence  than  a  dry  one. 


60  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

a  hand  duster  may  cover  from  two  to  ten  acres  per  day,  according 
to  the  size  of  the  trees  and  the  thoroughness  of  the  application.  With 
a  power  blower,  two  men  may  cover  from  20  to  30  acres  per  day,  pro- 
vided there  is  not  sufficient  wind  to  prevent  dusting  all  day.  Spraying 
is  very  much  slower,  the  range  being  from  two  and  one-half  to  six 
acres  per  day. 

A  serious  objection  to  the  dusting  method  is  that  two  or  more 
applications  are  usually  necessary,  even  where  success  is  obtainable  at 
all  in  this  way.  One  careful  application  of  a  liquid  spray  before  the 
attack  is  severe  at  any  point  should  give  a  long  period  of  immunity 
from  injury  and  is  frequently  sufficient  for  the  entire  season. 

Sulfur  Mixtures  for  Spraying 

Sulfur  may  be  applied  as  a  spray  in  a  pure  state  but  only  when 
combined  with  a  spreading  or  adhesive  substance,  such  as  casein* 
or  glue  water,  forming  the  so-called  "wettable  sulfurs"  which  are 
similar  to  the  commercial  sulfur  pastes.  It  may  also  be  applied  as 
a  caustic  spray  in  chemical  combination  with  other  substances,  for 
example,  a  lime-sulfur  solution  either  with  or  without  the  addition 
of  pure  sulfur.  Sulfur  applied  as  a  spray  has  apparently  the  same 
effect  as  when  applied  as  a  dust  and  adheres  better.  Its  action  is 
slow  but  extends  over  a  period  of  several  days. 

The  caustic  solution  of  lime-sulfur  acts  as  a  "contact  spray, ': 
killing  the  mites  within  24  hours,  provided  they  are  well  covered. 
This  spray  gives  immediate  relief  from  injury  and  hence  should  always 
be  used  when  the  mite  is  in  evidence.  A  spreader,  such  as  casein  or 
flour  paste,  is  especially  useful  with  all  caustic  sprays  to  facilitate 
wetting  all  the  foliage. 

Recommended  practice. — The  following  formulas  have  been  tested 
and  found  very  successful  for  the  past  two  years  at  the  University 
Farm  and  in  a  number  of  private  orchards  in  Sutter  and  other 
counties  of  northern  California.  To  get  the  best  results  they  must 
be  applied  at  the  right  time  and  with  painstaking  effort  to  cover  both 
the  upper  and  lower  sides  of  every  leaf.  Careless,  scanty,  or  hasty 
spraying  is  of  almost  no  value.  A  large  almond  tree  will  require  from 
eight  to  12  gallons  to  thoroughly  cover  it;  medium-sized  prune  trees, 
from  four  to  eight  gallons  per  tree. 

Sulfur  spray,  Formula  1,  is  to  be  applied  just  before  the  time  when 
the  mites  usually  began  feeding  in  the  orchard  in  previous  years,  but 
before  they  are  found  on  the  trees.  Should  the  work  be  delayed  until 
red  spiders  are  present,  then  use  Formula  2. 

*  There  are  now  other  forms  of  commercial  spreaders,  developed  since  this 
experimental  work  was  completed,  which  may  be  substituted  for  calcium  caseinate. 


BULLETIN  347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  61 

Formula  1 

Sulfur  (sublimed  or  powdered)    5  pounds 

*Calcium  casemate £  pound 

Water  to  make 100  gallons 

Prepare  a  paste  of  the  sulfur  and  casemate  as  directed  on  page 
62.  Add  this  to  the  spray  tank  with  the  agitator  running.  The  pres- 
ent commercial  form  of  casein  is  given  preference  as  it  is  a  better 
spreader  than  the  others  suggested  and  is  very  convenient  to  use. 

The  first  of  July  is  the  usual  date  for  this  application  in  the  Sac- 
ramento Valley.  It  maj^  be  necessary  to  apply  it  in  June  in  orchards 
where  the  spring  host  plants  are  very  abundant  or  if  drought  has 
hastened  the  death  of  the  first  host.  If  the  work  is  done  early  and 
thoroughly,  it  usually  holds  the  mite  in  check  for  the  entire  season. 

Contact  sprays. — Actual  infestations  by  the  common  red  spider  or 
the  brown  mite  require  a  quicker  acting  spray  than  sulfur  alone. 
A  spray  is  required  for  this  type  of  work  that  will  kill  the  mites  on 
contact  and  also  one  which  contains  sufficient  sulfur  to  act  upon  the 
3roung  mites  as  they  hatch  from  the  eggs.  The  following  formula  has 
been  found  very  satisfactory,  but  if  desired,  dry  lime-sulfur  or  other 
lime-sulfur  substitutes  may  be  used  in  place  of  the  solution.  These 
materials  are  added  in  the  liquid  form  after  the  paste  is  prepared  as 
under  formula  1. 

Formula  2 

Lime-sulfur  concentrate  (31°-34°  Baume)  on  almond 2  gallons 

prune 1  gallon 

peach 1  gallon 

Calcium  caseinate \  pound 

Sulfur  (powdered  or  sublimed)  5  pounds 

Water  to  make 100  gallons 

Heavy  infestations  of  red  spider  resulting  from  delayed  control 
cannot  be  entirely  checked  by  a  single  spraying.  One  application  will 
be  effective  for  about  three  weeks,  when  it  should  be  repeated,  unless 
the  attack  begins  very  late  in  the  season.  If  possible,  spraying  should 
never  be  delayed  until  injury  from  the  mites  is  noticeable,  as  it  is 
almost  impossible  to  get  as  good  results  from  late  spraying  as  if  the 
work  had  been  done  when  the  attack  usually  begins. 


*  The  glue-water  formula,  flour  paste,  or  other  commercial  spreaders,  may  be 
substituted  for  casein,  if  desired,  according  to  the  formulas  given  on  p.  62. 


62  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Caustic  sprays  are  more  dangerous  to  the  foliage  than  those  of 
sulfur  alone,  hence  particular  care  should  be  taken  to  work  at  tem- 
peratures below  100°  F.,  if  possible.  It  might  be  possible  in  limited 
areas  to  spray  when  the  sun  is  not  shining  directly  on  the  foliage  that 
is  being  drenched.  Much  stronger  concentrations  can  be  used  at  such 
a  time. 

It  has  been  reported  from  the  citrus  districts  of  southern  Cali- 
fornia that  the  use  of  caustic  sprays  has  killed  the  insects  that  feed 
upon  the  red  spider.  This  observation  has  not  been  substantiated  in 
northern  California;  moreover,  parasitic  or  predacious  enemies  are  of 
little  importance  at  the  height  of  the  mite  attack. 

Preparation  of  sulfur  pastes. —  (-a)  Casein  in  its  present  commer- 
cial form  of  calcium  caseinate  is  very  satisfactory  for  mixing  with 
sulfur  in  preparing  liquid  sprays.  It  aids  in  wetting  the  sulfur  and 
also  acts  as  a  spreader  and  adhesive  on  the  leaf  surface.  One-half 
pound  of  the  commercial  powder  is  sufficient  for  preparing  100  gallons 
of  mixture.  Add  the  casein  to  one  and  one-half  gallons  of  water,  into 
which  stir  five  pounds  of  sulfur  (powdered  or  sublimed)  until  a  stiff 
paste  without  lumps  is  formed,  adding  more  water  if  necessary.  Wash 
this  through  a  fine  screen  into  the  spray  tank  with  the  agitator  run- 
ning.   The  material  is  then  ready  for  application. 

(b)  The  glue-water  formula  suggested  by  Gray6  has  been  widely 
adopted  but  on  account  of  the  time  required  for  melting  the  glue  it  is 
less  convenient  than  the  casein  preparation.    The  formula  is : 

Formula  3 

Powdered  glue f  ounce 

Hot  water 1£  gallons 

Powdered  sulfur 5  pounds 

Water  to  make : 100  gallons 

After  melting  the  glue  in  hot  water,  it  is  diluted  to  one  and  one-half 
gallons  and  this  water  used  in  mixing  the  sulfur  into  a  paste,  free  from 
lumps.  Wash  the  paste  through  a  sieve  into  the  spray  tank,  while 
the  agitator  is  running.  For  small  amounts  of  this  mixture  the  more 
expensive  form  of  liquid  glue  may  be  substituted  for  the  dry  glue, 
thus  saving  the  time  and  work  required  in  melting  the  powdered  glue. 

(c)  Prepared  flour  pastes,  wet  (jelly)  or  dry,  such  as  bill  posters 
use  are  also  good  mediums  to  use  in  mixing  dry  sulfur  with  water. 
Home  made  flour  paste  (see  p.  70)  is  also  a  good  spreader  to  use 
with  caustic  solutions  of  lime  sulfur.  Two  pounds  of  the  powdered 
paste  or  three  pounds  of  the  wet  paste  (jelly)  is  sufficient  for  100 
gallons  of  mixture.  Flour  pastes  are  less  convenient  than  casein  in 
the  preparation  of  sulfur  mixtures. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


63 


(d)  All  mixtures  of  "wettable  sulfurs';i  should  be  used  the  day 
they  are  prepared.  If  allowed  to  stand  over  night  there  is  a  tendency 
to  '.'set"  and  form  a  solid  layer  which  is  difficult  to  bring  into  sus- 
pension. 

TABLE  VIII 

Efficiency  of  Sulfur  in  Different  Types  of  Liquid  Sprays  as  a  Control  of 
the  Brown  Mite  and  the  Common  Red  Spider 

(Applied  June   8   on  almond   trees.) 

(Amounts  used  are  for  100  gallons  of  spray  mixture.) 


Material  used 

Decrease  in  infestation* 

Plot 

No. 

observed 
6/18 

observed 
8/11 

observed 
9/20 

Remarks 

A 

Lime-sulfur  solution  (31°  Baume) 
2  gallons  (1-50)  plus  2  pounds  of 
dry  flour  paste  and  5  lbs.  of  flow- 
ers of  sulfur 

% 

96 

% 

90 

% 

90 

Control  satisfactory  for 
entire  season. 

B 

"Soluble  Sulfur"   (Niagra  Sprayer 
Co.)  3  lbs.  plus  2  lbs  of  dry  flour 
paste 

92 

60 

15 

Apparent  control  but 
results  not  permanent. 
Slight  foliage  injury. 

C 

Dry  lime-sulfur  (Shervvin  &  Wil- 
liams) 13^  lbs.  plus  2  lbs.  of  dry 
flour  paste 

85 

42 

5 

Control  unsatisfactory. 
Results  not  perma- 
nent. 

D 

Dry  lime-sulfur   (Sherwin  &  Wil- 
liams) 3  lbs.,  plus  2  lbs.  of  dry 
flour  paste 

93 

73 

25 

Apparent  control  but 
results  not  permanent. 

E 

"Milled  Sulfur"  (California  Spray 
Chemical  Co.)  5  lbs 

92 

88 

80 

Slower  in  action  than 
caustic  sprays  but  re- 

sults permanent. 

F 

Sulfur   (flowers  of   sulfur)    5  lbs. 
plus  2  lbs.  of  dry  flour  paste 

88 

88 

90 

Similar  to  No.  5. 

G 

Dry  lime-sulfur  (Sherwin  &  Wil- 
liams) 5  lbs.  plus  4  lbs.  of  wet 
flour  paste  and  5  lbs.  of  flowers  of 
sulfur 

95 

86 

90 

Equal  to  that  of  No.  1. 

H 

Dry  lime-sulfur   (Sherwin  &  Wil- 
liams) 3  lbs.  plus  4  lbs.  of  wet 
flour  paste  and  5  lbs.  of  flowers  of 
sulfur 

93 

85 

70 

Slightly    inferior    to 
No.  7. 

Check  (untreated). 


Percentage  of  live  mites  95  to  98. 


*  These  percentages  are  a  comparison  with  the  check  plots  in  which  the  number 
of  dead  mites  is  normally  from  2  to  5  per  cent. 


64 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


A  number  of  commercial  sulfur  pastes  are  now  on  the  market  which 
can  be  substituted  for  the  above  mixtures.  These  pastes  are  mixtures 
of  sulfur  and  other  substances  so  combined  as  to  give  a  wettable  sulfur 
with  adhesive  qualities. 


Fig.  8. — Almond  trees  at  the  University  Farm  sprayed  with  2  gallons  of  lime- 
sulfur  solution,  5  pounds  of  sulfur,  and  2  pounds  of  dry  prepared  flour  paste  in 
100  gallons  of  spray.  Sprayed  June  8,  photographed  August  4.  Compare  with 
Fig.  9. 

Experimental  spraying  at  the  University  Farm,  Davis. — Orchard 
and  laboratory  experiments  with  a  large  number  of  sprays  were  con- 
ducted at  Davis  and  in  the  immediate  vicinity  during  1920.  This 
work  was  done  principally  on  almonds  as  there  were  no  large  blocks 
of  prune  orchard  available.  The  more  promising  experiments  have 
been  tested  during  1921  on  prunes  at  the  University  Farm  and  in 
private  orchards  with  similar  results.  Hence  the  data  in  Table  VIII 
may  be  taken  as  indicative  of  the  behavior  of  such  sprays  both  on 
prunes  and  almonds. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


65 


The  plots  received  but  one  irrigation,  the  latter  part  of  July,  so 
that  all  suffered  from  drought.  The  number  of  trees  to  an  experiment 
ranged  from  14  to  40.  The  trees  are  about  the  size  of  the  average  full- 
grown  almond.    Dry  prepared  flour  paste,  such  as  bill  posters  use,  was 


\ 

4 

W   '      \  •X, 
\iXJ 


^A- 


Fig.  9. — Almond  trees  defoliated  by  the  red  spider,  T.  telarius.  These  trees 
were  grown  under  the  same  conditions  as  those  shown  in  Fig.  8  except  that  they 
were  unsprayed.     Photographed  August  4. 

combined  with  all  the  caustic  sprays  to  increase  their  spreading  quali- 
ties. The  sulfur  spray  in  plot  F  was  prepared  with  the  same  paste, 
instead  of  glue  water,  so  as  to  make  the  series  comparable.  The  amount 
of  spray  mixture  per  tree  was  from  8  to  12  gallons,  10  gallons  being 
the  average. 


66  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

These  applications  were  made  toward  the  close  of  the  brown  mite 
attack  and  just  as  the  common  red  spider  was  beginning  to  show  in 
the  orchard.  The  foliage  at  this  time  was  becoming  yellow  and  begin- 
ning to  drop,  the  combined  result  of  the  brown  mite  attack  and 
drought.  Within  ten  days  after  spraying,  the  plots  where  the  caustic 
solutions  were  used  (letters  A  to  D  inclusive,  and  G  and  H)  were 
regaining  their  normal  color.  Plots  E  and  F  did  not  recover  until  more 
than  a  month  after  treatment.  The  quick  action  of  the  caustic  solution 
resulted  from  its  effect  as  a  "contact"  spray.  A  large  percentage 
of  the  brown  mites  were  killed  within  24  hours  after  spraying  and  witli 
this  heavy  drain  removed  the  green  coloring  matter  was  quickly 
replaced  in  the  leaves.  The  sulfur  used  in  plots  E  and  F  acted  more 
slowly  but  just  as  surely,  so  that  in  the  final  summary  all  plots  which 
had  received  sprays  containing  approximately  five  pounds  of  sulfur 
per  hundred  gallons  were  almost  free  from  red  spider.  Plots  B,  C, 
and  D,  which  received  less  than  five  pounds  of  sulfur  per  hundred 
gallons  of  mixture  were  much  more  heavily  infested  from  August 
until  the  end  of  the  season.  It  would  seem  from  these  experiments  that 
the  red  spider  may  be  checked  on  almonds  by  a  single  thorough  spray- 
ing at  the  beginning  of  the  season.  The  type  of  spray  mixture  seems 
to  be  of  less  importance  than  the  amount  of  sulfur  used.  All  sprays 
containing  approximately  five  pounds  or  more  of  sulfur  per  100  gallons 
resulted  in  a  fair  degree  of  success. 

Sulfur  Dusting 

Powdered  or  sublimed  sulfur  has  frequently  proved  satisfactory 
as  a  control  for  the  different  species  of  red  spiders,  but  only  when  it 
adheres  well  and  is  in  close  proximity  to  the  mite.  Quayle  has  shown 
that  the  distance  over  which  it  is  effective  is  but  a  very  small  fraction 
of  an  inch.10  Lack  of  adherence,  poor  distribution,  and  the  heavy 
web  of  the  common  red  spider  are  the  causes  of  many  of  the  failures 
attributed  to  this  manner  of  application. 

Comparative  laboratory  tests  of  dry  and  liquid  applications  have 
shown  an  almost  uniform  efficiency  in  the  action  of  the  two  methods. 
These  tests  were  made  with  infested  twigs  sprayed  or  dusted  on  the 
same  day  and  in  the  same  way  as  the  work  was  done  in  the  orchard. 
The  twigs  were  then  placed  in  jars  partly  filled  with  water  and  each 
jar  set  in  the  center  of  a  sheet  of  tanglefoot  fly  paper.  The  latter 
precaution  prevented  the  loss  of  dead  mites  as  they  dropped  from 
the  twigs.  The  laboratory  temperature  was  practically  that  of  the 
orchard,  the  greatest  difference  in  the  two  situations  being  that  the 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  67 

laboratory  was  free  from  currents  of  air  sufficiently  strong  to  dislodge 
the  sulfur  from  the  leaves.  Under  these  conditions,  dusting  experi- 
ments requiring  thousands  of  mites  showed  an  efficiency  within  1  or 
2  per  cent  as  great  as  that  on  sprayed  twigs,  while  in  the  orchard  a 
difference  of  12  to  40  per  cent  was  common.  Sulfur  applied  as  a 
dust  in  the  orchard  apparently  does  not  adhere  sufficiently  long  to 
be  effective,  especially  with  the  little  pubescence  common  on  almond 
foliage. 

Control  by  dusting  seems  more  practical  for  the  brown  mite  than 
for  the  common  red  spider.  The  work  is  done  earlier  in  the  year 
when  the  humidity  is  higher  and  the  freedom  from  webbing  in  the 
case  of  the  brown  mite  makes  it  easier  to  bring  the  sulfur  in  intimate 
contact  with  the  mites.  The  most  successful  dusting  at  the  Univer- 
sity Farm  was  on  a  still,  foggy  morning  in  April  on  the  brown  mite. 
This  application  of  sulfur  adhered  through  a  three-day  heavy  north 
wind,  and  ten  days  after  the  application  there  were,  by  actual  count, 
from  81  to  102  thousand  grains  of  sulfur  per  square  inch  of  leaf 
surface.  This  one  application  controlled  the  brown  mite  for  the  entire 
season.  Other  applications  made  on  dry  windy  days  showed  scarcely 
a  trace  of  sulfur  24  hours  after  the  dusting.  Such  applications  are 
of  course  worthless. 

Sulfur  fillers. — Sulfur  for  dusting  purposes  must  be  sufficiently 
fine  so  that  it  will  adhere  well  to  foliage.  Sulfur  refiners  are  now 
preparing  a  very  finely  powdered  material  for  this  purpose,  but  such 
sulfur  used  alone  has  a  tendency  to  pack  and  become  lumpy.*  This 
material  does  not  spread  well  either  from  hand  or  power  blowers, 
unless  a  very  efficient  agitator  is  used,  as  it  has  a  tendency  to  clog  the 
machine  and  to  spread  very  unevenly.  This  difficulty  may  be  over- 
come by  mixing  an  inert  material,  such  as  hydrated  lime  or  kaolin, 
with  the  sulfur.  The  best  proportionate  amounts  of  sulfur  and  filler 
were  determined  in  the  experiments  reported  in  Table  IX.  This  work 
was  done  in  a  closed  laboratory  where  the  possibility  of  air  currents 
dislodging  the  sulfur  was  reduced  to  a  minimum.  Almond  twigs 
infested  with  red  spider  were  dusted  with  a  hand  blower,  and  placed 
in  jars  containing  water.  These  jars  were  set  on  sheets  of  tanglefoot 
paper  so  that  a  record  could  be  kept  of  the  mites  as  they  died  and 
dropped  off.  The  tests  were  made  in  duplicate  with  two  check  jars 
for  comparison. 

*  Eecent  developments  in  the  preparation  of  sulfur  dusts  are  overcoming  this 
trouble,  hence  it  may  not  always  be  necessary  to  use  fillers. 


68 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  IX 

Comparative  Efficiency  of  Sulfur  with  Varying  Amounts  of  Filler 
(Laboratory    test — applied    in    August) 


Proportions  of  sulfur 


(1)  Pure  sulfur 

(2)  90%  sulfur— 10%  hydrated 

lime 

(3)  80%  sulfur— 20%  hydrated 

lime 

(4)  70%  sulfur— 30%o  hydrated 

lime 

(5)  50%  sulfur— 50%  hydrated 

lime 

(6)  Check  (untreated) 


Per- 
centage 
of  red 
spiders 
killed 
August 
25 

Per- 
centage 

of 

mites 

killed 

August 

28 

90 

98 

90 

98 

85 

97 

77 

85 

65 

65 

95 

(alive) 

95 

(alive) 

Remarks 


Spreads  unevenly;  packs  on  standing. 

Spreads  more  evenly.  Requires  less 
material  to  cover  twigs  than  pure 
sulfur. 

Slow  action.  Effective  in  laboratory 
but  of  doubtful  value  in  orchard. 

Slow  action.  Inferior  to  larger  pro- 
portions of  sulfur. 

Inefficient. 


From  the  above  data,  it  is  concluded  that  dusting  sulfur  with  a 
ten  per  cent  filler  is  superior  to  the  pure  material  in  that  it  spreads 
more  evenly  and  gives  a  better  distribution  than  is  secured  with  pure 
sulfur.  There  was  little  difference  of  efficiency  between  the  pure 
sulfur  and  that  with  a  ten  per  cent  filler.  It  will  also  be  noted  that 
the  90-10  dilution  was  more  rapid  in  action  than  any  of  those  con- 
taining smaller  amounts  of  sulfur.  This  point  is  very  important,  for 
the  great  weakness  in  the  use  of  dusting  sulfurs  is  their  tendency  to 
shatter  off  from  the  leaves.  Hence  a  slow  acting  sulfur  may  not  be 
efficient  because  much  of  it  is  dislodged  before  becoming  effective. 
The  80-20  dilution  was  almost  as  efficient  as  the  90-10  after  the  lapse 
of  7  to  12  days,  but  for  shorter  periods  than  this  was  less  effective. 
In  orchard  practice,  therefore,  the  20  per  cent  filler  would  not  be 
desirable  unless  applied  when  the  leaves  were  damp  or  combined  with 
an  adhesive  material.  The  70-30  dilution  was  very  much  slower  in 
action  than  the  first  three  materials  tested,  and  the  50-50  dilution 
gave  such  poor  results  that  it  could  hardly  be  considered  worth 
applying. 

The  cost  of  dusting  material  with  a  ten  per  cent  filler  should  be 
about  the  same  as  the  cost  of  pure  sulfur.  It  is  doubtful  whether 
the  decreased  cost  obtained  by  using  20  per  cent  of  lime  would  be 
an  economical  practice,  unless  the  material  can  be  applied  under  the 
most  favorable  circumstances. 


BULLETIN  347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


69 


Field  tests  of  sulfur  with  varying  amounts  of  filler  confirmed  the 
laboratory  results,  viz.,  that  a  ten  per  cent  filler  increased  the  mechan- 
ical action  of  dusting  sulfur  without  a  loss  in  efficiency,  and  that  a 
50-50  dilution  was  decidedly  inferior  to  blends  with  higher  percent- 
ages of  sulfur. 

Intervals  between  dustings. — It  is  the  common  experience  in  sulfur 
dusting  that  the  applications  must  be  repeated  at  intervals.  The 
length  of  time  elapsing  between  applications  varies  with  the  adhesion 
of  the  sulfur  and  to  a  certain  extent  with  the  temperature.  At  75°  F. 
and  above,  the  effect  of  sulfur  is  more  toxic,  and  the  rate  of  hatching 
of  the  egg  is  shorter  than  at  70°  F.  and  below.  To  determine  the  exact 
intervals  between  spraying,  the  following  experiments  on  the  brown 
mite  were  made  on  almond  from  April  20  to  June  4.  The  sulfur  used 
was  a  mixture  of  a  special  dusting  brand  containing  a  ten  per  cent 
filler  of  hydrated  lime.  The  applications  were  made  at  intervals 
ranging  from  5  to  20  days  and  repeated  from  2  to  9  times.  All 
work  was  done  as  uniformly  as  possible.  The  sulfur  was  applied  with 
the  same  knapsack  blower  in  all  the  experiments,  the  work  being  done 
between  7  :00  a.m.  and  9  :00  a.m.  This  is  usually  the  stillest  time  of 
the  day  with  a  comparatively  high  humidity,  conditions  which  favor 
this  type  of  work. 

TABLE  X 

The  Number  and  Frequency  of  Sulfur  Dustings  Necessary  to  Control  the 

Brown  Mite   (Bryobia)   on  Almond 

(The  first  dusting  in  all  plots  was  on  April  20.) 


No.  of 
applications 

Days  elapsing 
between  dustings 

Observations 

No. 

May  17 

Percentage  of 

mites  dead 

June  14 

Relative  number  of 

live  mites 

% 

% 

1 

9 

5 

98 

1 

2 

5 

10 

96 

2 

3 

3 

15 

95 

12 

4 

3 

20 

70 

2 

5 

2 

5 

70 

35 

6 

2 

10 

65 

35 

7 

2 

15 

65 

30 

8 

2 

20 

70 

30 

Check 

(untreated) 

4 

96 

70  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

The  data  given  in  Table  X  show  that  at  least  three  dustings  are 
necessary  under  the  existing  conditions,  to  gain  a  satisfactory  control 
of  this  species  of  red  spider.  A  decided  decrease  in  the  degree  of 
efficiency  is  shown  in  all  plots  receiving  but  two  applications. 

A  fair  degree  of  control  is  seen  in  all  of  the  first  three  applications 
where  the  time  between  dustings  ranged  from  5  to  15  days,  but 
applications  at  20-day  intervals  were  inferior  to  those  of  15  days 
or  less. 

The  data  indicate  that  the  range  should  be  from  10  to  15  days 
rather  than  15  to  20  days,  with  3  as  the  total  number  of  applica- 
tions. The  sulfur  adhered  better  on  the  plots  dusted  early  in  the 
morning  than  when  applied  during  a  hot,  windy  time  of  the  day, 
particularly  in  midsummer.  Sulfur  in  noticeable  quantities  was 
present  in  plots  one  to  three,  inclusive,  throughout  the  entire  experi- 
ment. 

It  should  be  noted  that  this  work  was  done  on  the  brown  mite, 
which  is  not  a  web  spinner,  thus  making  possible  a  much  better  dis- 
tribution of  the  sulfur  among  the  mites  than  in  the  case  of  the  com- 
mon red  spider,  which  spins  a  heavy  protecting  web.  The  work  was 
also  begun  in  the  spring  when  the  humidity  is  higher  than  in  the 
summer,  consequently  the  sulfur  adhered  better  than  in  drier  weather. 
For  these  reasons  we  would  not  expect  so  good  control  of  the  com- 
mon red  spider  through  midsummer  as  is  shown  in  the  above  table. 

SUBSTITUTES  FOE  SULFUE 

It  has  long  been  recognized  that  there  is  an  element  of  danger  in 
an  indiscriminate  use  of  sulfur  on  certain  trees  and  plants.  The 
apricot  and  apple  in  certain  districts  are  usually  conceded  to  be  par- 
ticularly susceptible  to  "sulfur  sickness."  But  fortunately  the  stone 
fruits,  which  are  the  heaviest  sufferers  from  red  spider,  are  not  very 
susceptible  to  injury  from  this  chemical.  The  caustic  solutions  of 
lime  or  soda  and  sulfur  are  much  more  dangerous  to  foliage  than 
solutions  of  sulfur  alone.  But  the  application  of  weak  caustic  solu- 
tions is  seldom  attended  with  foliage  injury  unless  applied  at  tem- 
peratures of  100°  F.  or  more.  Even  in  such  cases  it  is  quite  probable 
that  the  injury  from  an  unchecked  attack  of  red  spider  would  be  very 
much  greater  than  that  resulting  from  spray  injury. 

Substitutes  for  sulfur  that  have  been  suggested  are  wheat  flour 
paste  (cooked),  consisting  of  one  pound  of  flour  to  one  gallon  of  water 
and  diluted  at  the  rate  of  one  part  of  paste  to  nine  parts  of  water.9 
The  time  and  work  required  to  cook  the  paste  have  been  a  great  deter- 
rant  to  the  use  of  the  formula,  but  prepared  flour  pastes,  which  are 


BULLETIN  347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


71 


sold  at  prices  comparable  to  that  of  a  good  grade  of  flour,  make  this 
material  of  practical  value.  Where  the  use  of  sulfur  is  attended  with 
considerable  risk,  flour  paste  mixtures  alone  might  be  used  with 
advantage. 

Spraying  with  water  is  sometimes  resorted  to  in  regions  where 
sulfur  applications  are  objectionable.  Such  sprays  are  of  value  in 
breaking  down  the  web  and  washing  large  numbers  of  the  mites  off 
the  leaves,  but  of  course  are  less  effective  than  those  containing  at 
least  one  gallon  of  lime-sulfur  solution  or  its  equivalents  (see  p.  74). 

Linseed-oil  emulsion  has  been  recommended  by  Vinal  particularly 
for  greenhouse  work.12  The  oil  is  emulsified  with  soap  and  used  at  the 
rate  of  one  or  two  gallons  per  100  gallons  of  spray.  This  mixture  is 
effective  on  cucumbers  and  violets  but  apparently  has  not  been  tested 
as  an  orchard  spray. 

Fish-oil  soaps  have  frequently  been  recommended  and  are  efficient, 
but  foliage  injury  frequently  occurs  from  their  use.  A  resin  wash 
has  been  highly  recommended  by  McGregor  for  use  in  South  Caro- 
lina,8 but  has  received  little  attention  in  California. 

None  of  these  materials  has  come  into  general  use,  perhaps  from 
inefficiency  or  lack  of  convenience  in  preparation.  Fish-oil  soap,  on 
account  of  its  variable  nature  and  the  danger  which  attends  its  use 
on  foliage,  cannot  be  recommended.  Nicotine  applied  in  the  sulfate 
form  has  frequently  been  suggested,  but  after  careful  tests,  was  found 
inefficient  both  as  a  liquid  spray  and  dust.  A  summary  of  the  results 
with  dusts  is  given  in  Table  XI.  Weak  emulsions  of  the  safer  forms 
of  petroleum  oils  are  also  being  used. 


.     '  TABLE  XI 

Experiments  with  Nicotine  Dusts*  on  Adults  and  Nymphs  of  the 

Brown  Mite 
(Laboratory  experiments  applied  May  8,  observed  May  13,  1920.) 


Material  used 

Number  and  percentage  of 
dead  mites 

Number  and  percentage 
of  living  mites 

2%  "Nicodust" 

480 

68.4% 
685 

87.2% 
610 

89.6% 

152 

(.8%  nicotine) 

5%  "Nicodust" 

31.6% 

88 

(2.0%  nicotine) 

4%  "Nicodust"  (1.6%  nico- 
tine) and  60%  Sulfur 

12.8% 
70 

10.4%. 

Check  (untreated) 

2% 

98% 

*  The  nicotine  dusts  used  in  these  experiments  were  prepared  by  the  California 
Walnut  Growers'  Association,  under  the  direction  of  Professor  Ralph  E.  Smith. 
They  are  composed  of  a  carrier  of  hydrated  lime  or  kaolin  combined  with  definite 
amounts  of  nicotine  sulfate.  The  percentage  of  nicotine  can  be  approximated 
from  the  amount  of  nicotine  sulfate.  For  example  a  2  per  cent  nicodust  is  sup- 
posed to  contain  0.8  per  cent  nicotine. 


72  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Banding  trees  with  tanglefoot  or  other  substances  which  might 
prevent  the  mites  from  crawling  up  the  trunk  has  proved  of  little 
value.  Such  measures  have  been  tried  repeated^,  but  with  so  little 
success  that  they  are  seldom  used. 

Natural  enemies. — Red  spiders  are  subject  to  attack  by  a  number 
of  insects,  including  predacious  thrips,  coccinellids.  a  coniopterygid, 
syrphid  fly  larvae,  and  predacious  bugs.  The  last  group  only  has 
been  recorded  at  all  commonly  during  the  height  of  the  attack  by  the 
common  red  spider,  and  then  in  too  few  numbers  to  be  of  practical 
value.  A  bug  (Triphleps  tristicolor)  has  been  noted  more  frequently 
during  the  summer  than  any  other  predator. 

The  mites  are  so  well  adapted  to  the  heat  and  aridity  of  the  interior 
valleys  during  the  summer  months  and  so  prolific  under  these  con- 
ditions that  no  natural  control  has  been  found  that  can  cope  with  them 
at  this  season.  Natural  control  by  parasitic  or  predacious  insects  in 
the  early  spring  seems  much  more  feasible.  The  mites  are  greatly 
reduced  in  numbers  during  the  winter  and  at  the  low  temperatures 
of  March  and  April  increase  very  slowly.  One  attack  on  the  common 
red  spider  at  this  time  of  the  year  was  noted,  apparently  the  work 
of  a  predacious  thrip,  in  which  the  mites  in  every  colony  examined 
were  killed  and  the  contents  of  the  eggs  eaten.  It  was  months  after- 
wards before  a  single  colony  of  mites  could  be  found  in  this  orchard, 
and  during  the  entire  year  the  red  spider  was  less  abundant  in  this 
orchard  than  ever  before  noted. 

II.     BROWN  MITE   (Bryobia  praetiosa  Koch.) 

Spraying  experiments  at  the  University  Farm  and  at  Durham 
show  that  the  most  effective  control  for  the  brown  mite  is  an  applica- 
tion of  crude-oil  emulsion  or  lime-sulfur  solution  for  the  winter  egg, 
during  December,  Januar}^,  or  the  first  of  February.  The  oil  emul- 
sions have  in  all  cases  killed  99  per  cent  or  more  of  the  winter  eggs  and 
the  efficiency  of  the  lime-sulfur  solution  has  averaged  95  per  cent. 

Crude-oil  emulsions  were  at  first  used  at  12  or  15  per  cent  strength. 
Later  experiments  have  shown  that  these  percentages  are  needlessly 
high  and  that  emulsions  containing  but  four  per  cent  of  oil  (four  gal-# 
Ions  in  100)  have  been  as  effective  as  an  ovicide  at  stronger  dilutions. 
The  latter  dilution  might  be  inefficient  against  insect  eggs  or  scale 
insects,  which  might  be  present,  hence  unless  the  object  is  to  control 
only  brown  mite  eggs,  it  would  be  better  in  most  instances  to  use  ten 
gallons  or  more  of  oil  in  100  gallons  of  mixture.     Two  disadvantages 


Bulletin   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


73 


TABLE  XII 

Comparative  Value  of  Oil  Emulsions  and  Lime- Sulfur  Solutions  as  a 

Control  of  the  Brown  Mite  (Bryobia  praetiosa)i  in  the 

Winter  Egg  Stage.     Orchard  Test* 

(Observed  3/17) 


Date 

Applied 

Material 
used 

Dilution 

No.  of 

trees 

sprayed 

Percentage 

of  eggs 

hatching 

Percentage 
of  hatched 

mites 

living 

Percentage 

of 
efficiency 

Remarks 

Crude  oil 

15  gals. 

% 

% 

% 

Shot  hole 

1-19-20 

emulsion 
(Home-made) 

of  oil 
in  100 

24 

1 

30 

99.7 

fungus 
unchecked 

Crude  oil 

15  gals. 

Shot  hole 

1-19-20 

emulsion 
("Ortho") 

of  oil 
In  100 

26 

1 

25 

99.75 

fungus 
unchecked 

Crude  oil 

15  gals. 

Shot  hole 

1-20-20 

emulsion 
("Buggo") 

of  oil 
in  100 

6 

1 

25 

99.75 

fungus 
unchecked 

Lime-sulfur 

10  gals. 

• 

Shot  hole 

1-20-20 

(Home-made) 
32°  Baume 

in 
100 

23 

50 

8 

96.0 

fungus 
checked 

Distillate 

QlA  gals. 

Shot  hole 

1-20-20 

Emulsion 

("Spra- 

mulsion") 

of 

emulsion 

in  100 

10 

14 

90 

87.4 

fungus 
unchecked 

Distillate 

Q%  gals. 

Shot  hole 

1-20-20 

emulsion 
("Zeno") 

of 

emulsion 

in  100 

12 

16 

90 

85.6 

fungus 
unchecked 

Distillate 

7  gals. 

Shot  hole 

1-20-20 

emulsion 
("  Distillate 
Emulsion") 

of 

emulsion 

in  100 

12 

18 

90 

83.8 

fungus 
unchecked 

Check 

Shot  hole 

(untreated) 

90 

92 

fungus 
unchecked 

*  Duplicate  orchard  and  laboratory  experiments  in  large  series  were  made  with 
the  materials  mentioned,  with  results  so  similar  that  the  above  data  may  be  con- 
sidered as  typical. 

t  Garman  reports  similar  results  on  the  egg  of  P.  pilosiis.50 


74 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


in  the  use  of  crude-oil  sprays  are,  first,  their  lack  of  fungicidal  action 
(for  example  on  "Shot-Hole  Fungus")  and  second,  their  somewhat 
low  efficiency  on  the  twig  borer  (Anarsia  linmtella).  These  faults  can 
be  largely  overcome  by  the  use  of  a  specially  prepared  crude-oil  emul- 
sion, which  can  be  safely  combined  with  lime-sulfur  solution.  This 
mixture  in  the  one  year  that  it  was  tested  showed  the  superior  qualities 
of  both  the  oil  and  lime-sulfur  preparations  at  the  dilutions  of  four 
gallons  of  crude  oil  and  one  gallon  of  lime-sulfur  solution  in  ten  of 
the  mixture. 

Proprietary  and  homemade  crude-oil  emulsions  proved  equally 
efficient.  A  satisfactory  type  of  oil  for  the  preparation  of  emulsions  is 
one  testing  18°  to  21°  Baume.  Very  light  crude  oil  was  slightly  less 
efficient  than  the  heavier  types. 


TABLE  XIII 

Comparative  Value  of  Lime-Sulfur  Solution   and   its    Substitutes*    and 
Crude  Oil  Emulsion  as  a  Control  of  the  Brown  Mite 
(Bryobia  praetiosa)  in  the  Winter  Egg  Stage 

(Laboratory  tests  applied  January  18,  1921.) 


No. 


1 
2 

3 

*4 

5 

*6 

7 
8 


Material  used 


Observed  February  23 


Percentage  of 

eggs 

hatching 


Lime-sulfur,  1  in  10 

Lime-sulfur,  1  in  10;  Nicotine 
sulphate,  1  in  800 

Barium-sulfide,    26  lbs.   in    100 
gals 

Barium-sulfide  (B.T.S.),  64  lbs. 
in  100  gals 

Dry  lime-sulfur,  20  lbs.  in  100 
gals ' 

Dry  lime-sulfur,  44  lbs.  in  100 
gals 

Crude  oil  emulsion,  4% 

Crude  oil  emulsion,  4%;  Lime- 
sulfur,  1  in  10 

Check  (not  sprayed) 


Percentage  of 

hatched 

mites  dying 


65 

70 

60 

55 

60 

65 
0 

4 
95 


10 
15 


0 

0 

5 
0 

10 
2 


Observed  March  1 


Percentage  of.  Percentage  of 


eggs 
hatching 


80 
95 
95 
65 

85 

90 

0 

12 
95 


hatched 
mites  dying 


100 
100 

91 

83 

81 
0 

100 
12 


*  The  substitutes  for  lime-sulfur  solution  are  here  compared  on  their  poly- 
Blllfide  content.  On  this  basis  it  requires  6.4  pounds  of  dry  barium-sulfld  or  4.4 
pounds  of  dry  lime  sulfur  to  equal  1  gallon  of  lime-sulfur  concentrate,  33°  Baume. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS 


75 


HOME-MADE  CEUDE  OIL  EMULSION 
Formula  4 

Crude   Oil - 4  to  12  gallons 

Fish  Oil  Soap   (hard) 4  to     7  pounds 

Fish  Oil  Soap  (liquid) 1£  to     3  gallons 

Lye  or  caustic  soda 12  to  16  ounces 

Water  to  make - 100  gallons 

No  injury  resulted  to  the  tree  from  airy  of  the  crude-oil  sprays, 
except  that  the  one  applied  just  before  blooming  slightly  injured  the 
tips  of  the  new  leaves.     It  will  probably  be  found  safer  to  apphr  oil 


Fig.  11. — Development  of  almond  buds  January  20,  when  the  second  series 
of  winter  sprays  was  applied  to  kill  the  winter  egg  of  the  brown  mite  {Bryooia 
praetiosa) . 


sprays  while  the  tree  is  quite  dormant.  Light  oils,  such  as  distillates, 
including  both  homemade  and  commercial  emulsions  and  those  listed 
in  Table  XII  as  "Zeno"  and  "  Spra-Mulsion, "  were  decidedly  inferior 
to  crude-oil  emulsion. 

The  time  for  applying  crude-oil  emulsions  is  during  the  dormant 
period  of  the  tree.  There  is  no  apparent  advantage  in  late  spraying, 
as  there  is  with  lime-sulfur  solutions.  Crude-oil  emulsions  have  been 
applied  during  December,  January,  and  the  first  week  of  February 
and  even  after  the  buds  were  opening  on  almonds,  without  any  injury 
to  the  tree.  It  is  probably  safer  to  spray  with  oil  on  almonds  and 
prunes  during  December  and  January  rather  than  later  in  the  year. 


76 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Avoid  spraying  with  oil  when  the  trees  are  very  dry,  for  example, 
following  a  heavy  north  wind.  At  such  time  there  is  greater  possi- 
bility of  absorption  of  oil  by  the  trees  since  the  amount  of  moisture 
in  the  twigs  has  been  reduced. 


TABLE  XIV 

Results  of  the  Combined  Effect  of  Sprays  and  Mite  Attack 

(Observed  May  21) 


Material  used 

Applied 

Months 

after 

application 

Percentage 

of 
decrease  in 
infestation 

Remarks 

Crude  oil  emulsion, 

1/19 

4 

98 

Foliage   a  deep   green,   except 

15% 

where  mite  is  present.     Side 

next  to   check  row  shows  a 

15%  increase  in  infestation. 

Lime-sulfur,  1-10 

1/20 

4 

92 

Foliage  a  lighter  green  than 
where  crude  oil  was  used,  but 
of  a  better  appearance  than 
early  spring  app^cations. 

Lime-sulfur,  1-50 

3/6 

2.5 

88 

Leaves  small;  beginning  to  fall. 

Distillate  oil,  7%  .... 

1/20 

4 

70 

Leaves  small  and  yellow,  com- 
parable to  check  row. 

Lime-sulfur,  1-50 

5/8 

0.5 

99 

Leaves  small  but  regaining  a 
deep  green  color. 

Check  (untreated).... 

3 

Leaves  small  and  vellow;  de- 

foliated very  rapidly. 

Lime-sulfur  solution  used  at  winter  strength  (one  in  ten  for  concen- 
trates testing  about  33°  Baume)  was  inferior  to  crude  oil  in  killing 
the  egg  of  the  brown  mite,  but  since  this  spray  is  also  of  value  in 
controlling  "Shot-Hole  Fungus"  {Cercospora  circumscissa)  and  the 
twig  borer  (Anarsia  lineatella) ,  when  applied  just  prior  to  blooming, 
its  total  value  in  most  orchards  might  be  greater  than  crude  oil.  The 
percentage  of  mite  eggs  actually  killed  by  the  lime-sulfur  solution  is 
low,  ranging  from  10  to  70  per  cent,  but  it  will  be  seen  from  Table  XII 
that  a  large  number  of  mites  which  hatched  died  within  a  few  days 
after  emerging.  The  total  efficiency  (i.e.,  including  both  the  eggs  and 
young  mites  that  were  killed)  of  the  lime-sulfur  spray  applied  at 
different  times  is  as  follows  : 

December  29,  1919,  total  efficiency 92.3  per  cent 

January  20,  1920,  total  efficiency 96.0  per  cent 

February  11,  1920,  total  efficiency 97.8  per  cent 

The  latter  date  is  from  seven  to  ten  days  before  blooming.  It  will  be 
seen  from  this  that  the  later  in  the  winter  this  spray  is  applied  the 
more  effective  it  is  in  control. 


Fig.  10. —  (Color  plate.)  Natural  color  of  foliage  restored  after  checking  the 
brown  mite  (Brijobia)  attack.  The  twig  at  the  right  was  photographed  thirty-six 
days  after  being  sprayed.  The  twig  at  the  left  shows  the  typical  color  of  the 
uiisprayed  trees.     Spray  applied  May  8. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  77 

Spring  and  early  summer  control  of  the  broivn  mite  is  best  effected 
by  the  caustic  spray  solution  as  recommended  for  the  common  red 
spider  (page  61).     Dusting  with  sulfur  is  more  effective  against  this 


Fig.  12. — Development  of  almond  buds  February  11,  when  the  last  series  of 
winter  sprays  was  applied  to  kill  the  winter  egg  of  the  brown  mite  (Bryobia 
praetiosa.) 


mite  than  against  the  common  red  spider,  but  is  slower  in  action  than 
the  lime-sulfur  solution.  The  almond  tree  responds  very  quickly  after 
the  mites  are  killed  and  within  a  month's  time  it  regains  its  normal 
color  (see  Fig.  10). 


78  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

III.     CITEUS  MITE  (Paratetranychus  pilosus  Can.  and  Fanz.) 

The  control  of  the  citrus  mite  on  deciduous  trees  is  similar  to  that 
of  the  brown  mite.  Winter  sprays  applied  to  the  egg  are  desirable 
but,  if  control  is  delayed  until  spring,  the  use  of  a  dilute  lime-sulfur 
solution  with  the  addition  of  five  pounds  of  powdered  sulfur  is  the 
quickest  means  of  control  (see  p.  61). 

SPRAY  PROGRAM  FOR  DECIDUOUS  ORCHARDS 

Almond: 

To  kill  the  winter  egg  of  the  brown  or  almond  mite  spray  in 
December  or  January  with  crude-oil  emulsion  4  to  12  per  cent  strength 
(formula  4,  p.  75). 

If  the  twig-borer  or  shot-hole  fungus  is  present  in  addition  to  red 
spider  eggs,  use  a  combination  of  crude-oil  emulsion  and  lime-sulfur 
(see  p.  74),  or  lime-sulfur  solution  alone  at  winter  strength  (or  its 
substitutes),  preferably  just  before  the  buds  open  (Table  XIII  and 
pages  73  and  76). 

For  the  spring  attack  of  the  brown  mite,  spray  with  lime-sulfur 
or  its  substitutes  (formula  2,  p.  61),  or  dust  every  10  or  15  days  with 
sulfur  (pp.  66-70). 

The  common  red  spider  is  controlled  in  the  same  way  as  on  the 
prune  in  the  interior  valleys. 

Prune: 

Interior  valleys. — Spray  all  orchards  subject  to  infestation  during 
the  first  two  weeks  of  July,  or  earlier  if  attacks  in  previous  years 
have  begun  before  this  date.  Do  not  wait  until  the  red  spiders  are 
seen  before  beginning  to  spray. 

Use  formula  1  (p.  61),  or  dust  carefully  three  times  (usually) 
every  ten  or  fifteen  days,  with  sulfur  containing  a  ten  per  cent  filler. 
The  latter  method  is  frequently  less  satisfactory  than  spraying, 
especially  if  much  web  has  been  spun  (pp.  59-60,  66). 

If  the  mites  are  well  established  in  any  part  of  the  orchard,  spray 
with  formula  2  (p.  61). 

Should  the  brown  or  almond  mite  attack,  use  the  methods  recom- 
mended for  this  mite  under  Almond. 

Coastal  regions. — Prunes  in  these  localities  are  more  subject  to 
attack  by  the  brown  mite  than  in  the  interior.  For  orchards  that  are 
commonly  infested  or  where  the  winter  egg  is  found  in  abundance, 
use  the  measures  recommended  under  Almond. 


BULLETIN  347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  79 

The  common  red  spider  is  not  so  troublesome  as  in  the  interior 
valleys,  but  may  require  control,  especially  in  dry  years.  For  this 
mite  use  the  same  sprays  as  in  the  interior,  though  dusting  is  more 
successful  in  these  localities  than  in  the  interior  valleys. 

Peach : 

Red  spiders  seriously  injure  peaches  in  parts  of  the  interior,  par- 
ticularly the  San  Joaquin  Valley.  This  injury  is  principally  due  to 
the  common  red  spider.  Use  the  control  measures  recommended  for 
prunes  in  interior  valleys  (p.  78). 

COST  OF  SPRAYING   AND    DUSTING 

Spraying.— To  thoroughly  wet  a  full-grown  tree  in  foliage  will 
require  from  7  to  11  gallons  of  spray,  the  latter  amount  for  large 
trees  like  almonds.  Allowing  70  trees  to  the  acre  and  an  average  of 
9  gallons  per  tree,  630  gallons  per  acre  would  be  required.  With 
sulfur  at  5  cents  a  pound,  lime-sulfur  solution,  20  cents  a  gallon, 
and  calcium  casemate,  23  cents  a  pound,  the  cost  of  the  materials  in 
100  gallons  of  spray  and  per  acre  is  as  follows : 

Per  100  gallons 

Formula  I $0.36 

Formula  II 0.56  to  0.76 

Formula  III 0.26 

Formula  IV 0.82  to  1.48 

The  cost  of  application  fluctuates  with  the  price  of  labor  and  the 
type  of  apparatus  used.  A  power  sprayer  requires  a  team  and  two 
or  three  men  to  operate  it,  according  to  whether  a  "spray  gun"  or 
the  regular  nozzle  is  used,  and  possibly  the  cost  of  a  supply  wagon 
must  be  included.  The  amount  of  spray  applied  per  day  varies  from 
800  to  1800  gallons  per  day. 

Dusting.  Knapsack  Duster 

Sulfur  (15  to  20  pounds  per  acre)  $0.75  to  $1.00 

Labor  cost  (1  man,  $2.50  per  day)  (2  to  10  acres  per  day)  0.25  to     1.25 


Per 

acre 

$2.28 

3.58 

to 

4.84 

1.67 

5.20 

to 

9.37 

'Range  of  total  cost  per  acre  for  labor  and  materials $1.00  to  $2.25 


*  Acreage  covered  based  on  trees  of  five  years '  age  or  more.  It  should  always 
be  noted  in  the  comparison  of  the  cost  of  dusting  versus  spraying  as  a  control  for 
red  spider,  that  for  most  situations  it  is  necessary  to  make  two  or  three  applica- 
tions of  dust  to  one  of  a  liquid  spray,  and  even  then  the  repeated  dustings  may 
be  less  efficient  than  one  thorough  spraying. 


80  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Power  Duster 

Sulfur  (20  to  Qo  pounds  per  acre)  _—       $1.00  to  $3.25 

Labor  cost   (2  men  and  1  team  per  day,  $5.00) 
(Machine  operation,  per  day,  $1.00) 

$9.00 
Labor  per  acre  (20  to  30  acres  for  an  all-day  run)  0.30  to     0.45 


Range  of  total  cost  per  acre  for  labor  and  materials $1.30  to  $3.70 

SUMMARY 

Three  species  of  red  spiders  are  common  in  deciduous  orchards : 
the  common  red  spider  or  two-spotted  mite  which  attacks  the  trees 
during  midsummer ;  the  brown  mite  or  almond  mite,  which  passes  the 
winter  in  the  egg  stage  on  the  tree,  feeds  from  the  first  of  March  to 
August ;  and  the  citrus  red  spider,  also  found  on  the  tree  during  the 
winter  in  the  egg  stage. 

The  common  red  spider  winters  in  a  dormant  condition  or  feeds 
upon  hardy  weeds  and  cultivated  ^plants.  There  is  a  slow  increase  in 
numbers  during  the  spring  on  wild  morning-glory,  Malva,  and  other 
weeds.    Migration  to  orchard  trees  occurs  in  June  or  the  first  of  July. 

Red  spider  attack  results  in  pale,  yellow  leaves,  followed  by  defolia- 
tion. Trees  so  affected  are  incapable  of  producing  the  required  food 
for  maturing  the  year's  crop  and  buds  for  the  following  year. 

Red  spider  attack  decreased  the  crop  value  in  two  prune  orchards, 
$113  and  $453  per  acre,  respectively.  This  included  loss  in  weight  of 
crop  and  reduction  in  grade. 

Drought  is  usually  associated  with  red  spider  injury.  Thrifty  trees 
with  abundant  moisture  are  less  liable  to  this  type  of  loss,  but  ample 
soil  moisture  is  not  absolute  protection  against  red  spider  attacks. 

Timely  irrigation  or  the  control  of  either  the  spring  or  summer 
attacks  of  red  spiders  caused  a  marked  increase  in  the  number  and 
the  size  of  fruit  buds  in  orchards  attacked  by  red  spider. 

Spraying  as  a  control  for  the  common  red  spider  should  be  done  at 
the  time  the  attack  usually  began  in  previous  years,  even  though  the 
mites  cannot  be  seen.  The  spraying  must  be  done  very  carefully, 
using  five  pounds  of  sulfur  (made  into  a  paste  with  calcium  casemate 
or  glue  water)  to  100  gallons  of  water. 

Should  the  spraying  be  delayed  until  the  red  spider  is  abundant, 
two  or  more  applications  at  three  weeks'  intervals  are  usually  neces- 
sary. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  81 

Sprays,  applied  after  the  common  red  spider  is  numerous  in  the 
orchard  or  for  severe  attacks  of  the  brown  mite  or  citrus  mite,  should 
contain  one  gallon  of  lime-sulfur  concentrate  (or  its  substitutes)  to 
100  gallons  of  mixture. 

The  control  of  any  species  of  red  spider  by  spraying  requires 
timely  and  extremely  careful  applications.  Careless,  scanty  spraying 
is  almost  worthless.  Cover  both  sides  of  the  leaves  on  every  part  of 
the  tree,  using  from  five  to  ten  gallons  per  tree.  An  early  applica- 
tion, usually  about  the  first  of  July,  is  of  far  more  value  than  late 
sprayings. 

Sulfur  dusting  is  of  the  greatest  value  in  regions  of  light  wind  and 
moderate  humidity.  Use  a  10  per  cent  filler  of  hydrated  lime  in  all 
dusting  sulfurs  and  apply  every  10  or  15  days.  Three  applications 
are  usually  necessary  unless  all  conditions  are  very  favorable. 

The  brown  mite  is  most  easily  controlled  in  the  egg  stage  by  winter 
spra}Ts  of  crude-oil  emulsion  or  lime-sulfur  solution,  winter  strength. 
The  oil  emulsion  may  be  applied  at  any  time  during  the  winter,  but 
lime-sulfur  is  more  effective  as  the  cluster  buds  are  opening. 

Lime-sulfur  in  the  dry  or  solution  form  is  also  of  value  in  control- 
ling the  peach  twig  borer  and  shot-hole  fungus.  If  the  latter  pests  are 
present,  it  is  better  to  use  lime-sulfur  than  crude-oil  emulsion,  even 
though  the  latter  is  more  effective  against  the  egg. 


82  UNIVERSITY    OP    CALIFORNIA EXPERIMENT    STATION 


BIBLIOGRAPHY  OF  LOCAL  INTEREST 

i  Banks,  Nathan. 

1900.     "The  Eed  Spiders  of  the  United  States."     U.   S.  Dept.  Agr.,  Div. 
Ent.,  Tech.  Ser.  8,  pp.  65-79,  figs.  1-16. 

2  DE  Ong,  E.  R. 

1918.  "Control  of  Red  Spiders."     Mo.  Bull.  Calif.  St.  Com.  Hort,,  vol.  7, 

no.  3,  pp.  111-118. 

1919.  "The  Red  Spider."     Mo.  Bull.  Calif.   St.   Com.   Hort.,  vol.    8,   nos. 

11-12,  pp.  679-680. 

1921a.  "Controlling  the  Red  Spider."  In  Univ.  Calif.  Jour.  Agr.,  vol.  7, 
no.  1,  pp.  3-4,  23-24. 

1921b.  "Suggestions  for  the  Control  of  Red  Spiders  in  Deciduous  Or- 
chards." Mo.  Bull.  Calif.  St.  Dept.  Agr.,  vol.  10,  nos.  5-6,  pp. 
186-191. 

1922.  <  <  Summary  of  Red  Spider  Control. ' '  Mo.  Bull.  Calif.  St.  Dept.  Agr. 
(in  press). 

s  Essig,  E.  O. 

1913.  "Injurious  and  Beneficial  Insects  of  California."     Mo.  Bull.  Calif. 

St.  Com.  Hort.,  vol.   2,   nos.   1   and  2.     Reference  to  red  spiders, 
pp.  6-10,  figs.  9-11. 
1915.     "Injurious   and   Beneficial   Insects   of   Calif."      Supplement   to   Mo. 

Bull.  Calif.  St.  Com.  Hort.,  vol.  4,  no.  4,  pp'.  13-17. 
1922.     "The  European  Red  Mite."     Mo.  Bull.  Calif.  St.  Dept.  Agr.,  vol.  11, 

no.   4,   pp.   409-411. 

*  Ewing,  H.  E. 

1914.  "The  Common  Red  Spider  or  Spider  Mite."     Oregon  Agr.  Exp.  Sta., 

Bull.  121,  95  pp. 
1921.     "New  Nearctic  Spider  Mites  of  the  Family  Tetranychidae. "     Proc. 
U.  S.  Nat.  Museum,  no.  2394,  vol.  59,  pp.  659-666. 

s  Garman,  Philip. 

1921.     "The  European  Red  Mite  (Paratetraiiychus  pilosus  Can.  &  Fanz.)  in 
Connecticut. ' '     In  Jour.  Econ.  Ent.,  vol.  14,  no.  4,  pp.   355-359. 

5°  Garman,  Philip. 

Same,    hi  Report  Conn.  St.  Entomologist,  1921,  pp.  146-152. 

6  Gray,  Geo.  P. 

1918.  "Wettable  sulfurs."  Mo.  Bull.  Calif.  St.  Com.  Hort.,  vol.  7,  no.  4, 
pp.  191-192. 

7  Harvey,  E.  M.,  and  Murneek,  A.  E. 

1921.  "The  relation  of  Carbohydrates  and  Nitrogen  to  the  Behavior  of 
Apple  Spurs."     Oregon  Agr.  Exp.  Sta.  Bull.  176,  47  pp. 


BULLETIN   347]       CONTROL  OF  RED  SPIDERS  IN  DECIDUOUS  ORCHARDS  83 

s  McGregor,  E.  A. 

1916.  "The  Red  Spider  on  Cotton  and  How  to  Control  It."     U.  S.  Dept. 

Agr.,  Farmers'  Bull.  735,  12  pp.,  10  figs. 

McGregor,  E.  A.,  and  McDonough,  F.  L. 

1917.  "The  Red  Spider  on  Cotton."    U.  S.  Dept.  Agr.  Bull.  416,  72  pp. 

9  Parker,  W.  B. 

1913.  "The  Red  Spider  on  Hops  in  the  Sacramento  Valley  of  Calif ornia. ' ' 
U.  S.  Dept.  Agr.,  Bur.  Ent.,  Bull.  117,  41  pp.  9  figs.,  6  pis. 

1913.  "Flour  Paste  as  a  Control  for  Red  Spiders  and  as  a  Spreader  for 
Contact  Insecticides. ' '  U.  S.  Dept.  Agr.,  Bur.  Ent.,  Circ.  166, 
5  pp.,  2  figs. 

io  Quayle,  H.  J. 

1912.  "Red  Spiders  and  Mites  of  Citrus  Trees."     Univ.  Calif.  Agr.  Exp. 

Sta.,  Bull.  234,  pp.  483-530,  35  figs. 

1913.  "Some   Natural  Enemies   of   Spiders   and  Mites."     In   Jour.   Econ. 

Ent.,  vol.  6,  no.  1,  pp.  86-88. 

ii  Stabler,  H.  P. 

1913.     "Red  Spider  Spread  by  Winds."     Mo.  Bull.  Calif.  St.  Com.  Hort., 
vol.  2,  no.  12,  pp.  777-780. 

12  Vinal,  Stuart  C. 

1917.  "The  Greenhouse  Red  Spider  Attacking  Cucumbers  and  Methods  for 
Its  Control."     Mass.  Agr.  Exp.  Sta.,  Bull.  179,  p.  33. 

is  Volck,  W.  H. 

1903.     "Sulfur    Sprays   for    Red   Spiders."     Univ.    Calif.    Agr.   Exp.    Sta., 
Bull.  154,  10  pp. 

1913.  <  <  The  Control  of  Red  Spiders. ' '  Mo.  Bull.  Calif.  St.  Com.  Hort.,  vol. 
2,  nos.   3-4,  pp.   356-363. 

14  Written,  J.  C. 

1919.     "Frost  Control  and  Related  Factors."     Mo.  Bull.  Calif.  St.  Dept.  of 
Agr.,  vol.  8,  no.  11-12,  pp.  675-678. 

15  WOODWORTH,   C.   W. 

1902.  "The  Red   Spider   of  Citrus  Trees."     Univ.   Calif.  Agr.   Exp.   Sta. 

Bull.  145,  19  pp. 

1903.  Entomology.     In  Univ.  Calif.,  Agr.  Exp.  Sta.,  Bpt.,  1901-1903,  pp. 

104-110.      (Red  Spider  Remedies,  p.  105.) 


